RV Tire Safety

Is it OK to change tire size on duallies?

2026-04-03T10:10:00.000-07:00

Question from RVer on a forum

“I need six new tires for a 2004 38-foot Dutch Star on a Spartan chassis. Michelin tire models and sizing have me totally confused. Current Michelin size is 275/80R22.5. Dealers claim this Michelin size is interchangeable to 295/75R22.5 from brands that don’t offer the original size. They tell me there will be no ill effects on the drive line or instrumentation.”

My response:
I was concerned with this possibly incorrect information. In this case, the two sizes have the same load capacities, so that was not my concern.

Some folks replied to the original question on the forum by suggesting he use this tire size website or similar websites to learn about tire sizes. I did have a little problem with using such a “tire size” site, as the information is aimed at car applications and does not include special information regarding dual-tire applications. Also, the resulting numbers may or may not match published industry standards.

I pointed out my concern about the Minimum Dual Spacing (MDS), which is a published dimension in industry standards and has been covered previously in this blog.

Some folks reported that they have changed tire sizes and had no problems, implying that any change might be OK

I provided this graphic to help people understand what this dimension is:

As you can see, the wheel offset and even wheel thickness can have an effect on this spacing. I also pointed out this example: 275s have an MDS of 12.24, and 295s have an MDS of 13.19.

Let's assume that with your wheels and with the 275s you have a physical 1.00" clearance down at the bulge in the sidewall near the road. So if you don't change wheels and put wider 295s on the new, clearance will be 1.00 - ( 13.19 - 12.24) or 1.00-0.95 or a final clearance of 0.05", which is clearly too small.

I hope that those considering changing tire size consider not only the load capacity, which is very important, but also the dimensions.

Problems with small trailer tires.

2025-11-18T10:20:00.000-08:00

Roger, back in March 2024 I bought a Bear Track Aluminum Trailer to tow my new-to-me Harley Trike. The bed is 10', trike fits great, trailer weight 400 lbs and I love it. But not so happy about the tires. For one thing, I just found that when I bought the trailer, the tires were already over 2 years old, date stamp is "0822". Anyway, we did a short trip in Apr 2024, all was fine. Then a big trip in Oct 2024 and about 80 miles from home we had a flat. A puncture, so not the tire's fault. But then in March 2025 I took the trailer back to where we bought it, because one side the wheel wiggled. They said the bearing needed tightened. They did that under warranty, but also told me the tire on the other side had a broken belt. They got that replaced under warranty as well. Then just last week, returning home from a 5 day trip, the other tire a belt broke while on the road and the tire went immediately flat. So now, all 3 tires, only the patched tire (now the spare) is even usable.

So my question is, what brand do people recommend most for trailer tires and where can I get some that were made very recently. The ones that came with the trailer are MasterTrack UN203 Branded, Size: ST175/80R13, other numbering: 6PR 91/87M Load Range C, rated at 1360 lbs (single) I think that means one tire on each side. So the Trike weight 1200 lbs and the trailer 400 lbs. So that should be 1600 lbs total or only 800 lbs exerted onto each tire. Well below the limit indicated.

Also this tire says Plies: Tread Polyester 1 + Steel 2 Sidewall Polyester 1

I am willing to spend more for a better brand, higher Load Range, whatever it takes.

My answer..............

Based on the numbers in your post, I'm not sure why you are having "broken belt" problems. That term is kind of a catch-all, so without seeing the tire and lacking a few pictures of the "Broken Belts", I am doing a little "shooting in the dark, but here goes.

An 0822 date tire should not be having belt problems at that light a load. However if the dealer was willing and able to change one tire under warranty at 3 years old and the second tire also having some issue at 3-1/2 years, it is not unreasonable for you to "shop around".

IMO an ST175/80R13 LR-C when only loaded to 50% of its "rated load" in your type of service should be good for 4+ years assuming it is always fully inflated to 50 psi.

I would suggest you check with etrailer.com as they have both radial and bias tires in LR-D in your size. Even though the etrailer tires are LR-D you can get 1,360# load capacity at 50 psi as long as you keep them inflated to that level.

Tire terminology: Basic tire construction, Part 1

2025-10-13T17:38:00.000-07:00

After a recent post on selecting travel trailer tires, I received a question from a reader who asked, "What is a cap ply?" I sent an email explaining that some tires include a layer of fabric on top of the belts to help stabilize the tread area of the tire. Pay attention to the different types of tire. Some have a "cap ply" and some do not.

You can learn if a tire has this reinforcement if you "read" the tire sidewall. For example, the tire in this picture has 2 body Polyester ply + 2 steel ply + 2 Nylon Ply in the tread area. It also has 2 ply of Polyester in the sidewall.

Obviously, I am too close to the topic of tire construction and sometimes too focused on the technical terminology, so I decided to cover some of the basics. It is easiest to provide links to a few videos on tire construction. But first, I want to be sure that we are all on the same page.

1. Different tire applications, i.e., trailer, car, pickup, small motorhome, and Class A RVs all have different loading and different needs, so there are some construction differences among the tire needs for each application.

2. Having said that, tires for different applications are "different." But it is also important to understand that the general features and parts of tires are similar. So, don't jump over one video just because the tires in that video are not specific to your use. There are still enough similarities that some of the information presented will help you develop a better understanding of the features in your tires.

Basic passenger car tire

Let's start with a basic passenger car tire with a video from the U.S. Tire Manufacturers Association. This tire has 1 body ply and 2 steel belts and no "cap ply". It is just your basic, low-cost passenger car tire. The steps in this video would be what you saw if you were able to visit any tire plant in the world.

You will note that almost all radial tires, from a small 13" to a large 24.5" tire, have the basic features and components shown in the video, those being inner liner, body ply, sidewall, belts, and tread.

High-performance passenger car tire

The first video was an animation. Now, let's look at the real thing, in this case, a high-performance passenger car tire. Did you note the addition of rubber-coated fabric (time 3:15 in the video) just before the tread rubber was applied?

This is what is known as a "cap ply" or tread "overlay". The "cap ply can be the full width of the belts, as seen in this video. Or, it may be just in the center portion of the tread or possibly two individual strips of reinforced material over the edges.

Different tires have different performance needs, and different tire companies can select different methods of meeting the performance, durability, and cost objectives for their tires.

Next week we will cover larger, all steel radials, as found on most Class A RVs.

If you have any questions on this information you can email me at Tireman9 (at) gmail.com or use the form below.

Why do radial tires fail when overloaded or underinflated?

2025-10-12T15:17:00.000-07:00

I have a number of posts here on the topic of why tires fail. There are also more than 16 posts that mention "separation" or more accurately a "Detachment", as a reason or contributor to a tire failing. But maybe, some readers want more a scientific explanation.

This graphic is a computer-generated color depiction of the relative heat generated internally in a tire as it is driven down the road. Light blue is the coolest area, and red is the hottest.

When tires crack

I have covered the "artificial aging" of rubber in several posts. Basically, the molecular cross-link density increases, which can be thought of as stiffening and less flexibility. When rubber loses its flexibility, the molecular bonds crack. As more bonds crack, these cracks join up until they can be seen with the naked eye, as pointed out with the yellow arrows in the picture below.

When tires develop these larger cracks, they simply grow over the next hundreds or thousands of miles, as seen in the next picture.

"Free-Spin Inspection"

Tires with these larger cracks can be identified by doing a "Free-Spin Inspection," as covered in detail in this post on how I inspect my tires.

When doing a "Free-Spin Inspection", If you discover the out-of-round or side-to-side movement as seen in the video in the above post, I strongly suggest you have the tire inspected by your tire dealer for that brand tire, or simply replace the tire if it is more than 3 years old.

By the way, the steel belts do not "slip". They are no longer attached as part of the structure of a radial tire.

I hope this helps understand the situation.

Does your tire says "MAX PSI 80 psi." or something similar?

2025-09-04T18:13:00.000-07:00

My answer is that while those words or similar may be on the sidewall, those words are NOT to be taken literally as an instruction because those words are in reality only PART of an important phrase.

In reality your tires more likely say something like "Max Load 2500 pounds at 80 PSI Max Inflation". Of course different vehicles will have different size and type tires but we are looking at some general guidelines here.

Do you see and understand that the actual information the tire company is telling you is that the tire has a maximum load capacity of 2,500 pounds when it is inflated to 80 Psi? Most people understand that the load capacity of a tire is controlled and limited by the inflation in the tire, so this phrase is telling us that the load capacity of the subject tire will never be greater than 2,500 pounds even if you increase the inflation pressure

The loads and inflation are published in industry standards books such as the US Tire & Rim Association. Almost all tires made for sale and use in the US will follow the industry standards book AKA "TRA".

I believe that the RV company engineers either have copies of the books or at least copies of the applicable pages.

As always, you need to remember we are ALWAYS talking about the "cold" inflation which means when the tire is at Ambient air temperature and has not been driven or in direct sunlight for the previous couple of hours. If I ever talk about the warm or hot inflation that will be made obvious in that post.

Every vehicle has a Certification Label. This image is from a Class-A RV.

The specifics of this individual label are not important as you should have captured a nice, sharp picture of the label for your RV. Class-A has the label usually near the driver's left elbow, Class B & C and LT have the label in the driver door jam and towables (TT and 5th wheel trailers) have the label on the outside, driver side, toward the front of the trailer.

The label states the Tire Size, and the recommended inflation if it is a passenger car. If it is an RV it will give the GAWR, Tire Size, Tire Load Range, and the inflation required to support the GAWR. Passenger car and pick-up truck inflation is arrived at after years of testing and evaluation of different tire designs from different manufacturers at different inflation that balance the requirements on a long list (hundreds of items) from the vehicle company. These requirements include Force & Moment test results and fuel economy plus dozens of detailed ratings within each category for Ride, Handling, and Noise. Car inflation is a "Recommendation" from the engineers at the car company aimed at giving you the performance they designed into the car.

I have never heard of RV companies evaluating tires. It seems that they generally look for the lowest-cost tire that can provide the load capacity required by law. This usually means for towables that the tires would need to be inflated to the level required to deliver the max load capacity. A few RV companies are selecting better tires with Nylon Cap Ply but you need to pay attention.

The inflation number on a tire sidewall is the PSI needed to deliver the Max Load rating stated on the tire. Normally tire load capacity is increased with an increase in inflation and that would normally mean you would increase the inflation, BUT since the tire was selected for its max load capacity as stated on the tire sidewall, the inflation that will deliver the highest load capacity for that tire is the "MAX infl" which is NOT the highest inflation the tire can tolerate.

Yes, the wording can be confusing but the wording was not selected with the average user in mind.
Motorhomes are built on chassis built and designed by vehicle manufacturers so the tires on Motorhomes will many times have inflation on the label that is lower than the tire max because the tires were evaluated and selected by the vehicle chassis manufacturer.

Many towables are built with minimal capacity for additional "stuff" but many people load their RV with no thought to the load capacity. This is why the data shows that over half the RV on the road have one or more tire or axle in overload. This is a major contributor to tire failures and why there are so many RV trailer tire failures.

Too often on RV forums, people do not pay attention to what type of vehicle is being discussed A Passenger car (or 1/2 tom Pickup) or a Motorhome or a Trailer (5th wheel). These three completely different types of vehicles require three different guidelines and inflation information. This failure to be specific with the vehicle type is why people get confused.

How to avoid Tire Valve Stem failures

2025-08-20T14:56:00.000-07:00

I received a question:

"I pull a 27 foot travel trailer with dual axles and carry two spare tires and I needed both of them. One is on the bumper and the other is on an under-mount. I do not use a TPMS because we had one and had issues with valve stem leakage.'

I have responded to similar questions and provided links to one or more of the dozen posts I have made in this blog on the topic of "Tire Valves".

I guess that some folks did not check these resources, so here is what I hope is a comprehensive reply to the issue of tire valve failure when TPM sensors are attached to standard rubber stems.

If you had problems with valve stem leaking, then it is most likely that you were using the "low-cost" rubber stems intended for passenger car tires, like a TR413 as seen here:

TPMS external sensors screw on the end of the stems, and while they are not very heavy, that extra bit of weight does result in vibration, which can result in the fatigue failure of the rubber stem as seen here.

Here is an example of the movement of a rubber stem with an external:flow-thru" TPMS.

There are some rubber valves called "Hi-Press" as they are "rated" for 80 psi.
Here is a short video showing a FAILED TR800HP valve stem.

[video width="406" height="720" mp4="https://www.rvtravel.com/wp-content/uploads/2025/08/3-HP-rubber-valve-fail-due-to-TPM-sensor.mp4"][/video]

Here is a comparison of a few different stems.

From the left, we see a TR800HP that some call "Hi Press", next a TR413, then TR416 and TR435, which are both "bolt-in" stems and will not vibrate and fail due to flexing. Note: I placed pennies under 3 of the stems to raise the shoulders for them all to the same level for the picture. I wanted the comparison to be as if the valves were at the same level in the wheel.

Auto Zone has TR416 bolt-in stem Part # 20128 SKU #582043 at $6.99 for 2.

O'Reilly stores carry Part # 15-4559-2 1" chrome Tire Valves" - Fits Rim Hole Diameters of 0.438" and 0.625 Inch. The list price is $7.49 for 2.

Many other auto parts stores carry similar "bolt-in" stems

Some Aluminum wheels have a deep recess for the valve, as seen in this picture.

I understand that some Airstream RVs and some Ford vehicles have this type of bolt-in valve. It is a "Dill-VS-902-W" and can be found with a quick Internet search for $2.50 to $6.50. I have read reports that some "Discount Tire" stores have or can get these Dill stems.

NOTE: There are 2 different hole diameters in the wheels. Most ST tires and LT tires will come on wheels with the smaller 0.375" dia hole. The larger 0.625 gasket will not fit in the smaller hole. Both parts listed above show both sizes of rubber grommets. Select the appropriate size for your wheels.

This shows the two different grommets, so its pretty hard to get the wrong one. It is a light press fit or even "slide-in' fit into the hole in the wheel.

Special Caution. All of these "Bolt-In" valve stems need to be tightened to 25 - 45 INCH- POUNDS, so don't over-do it.

I believe that if you switch to "Bolt-In" stems, you will avoid the vibration failure of the stems. I always use "bolt-in stems" in my race cars and on my RVs and trailers and have never had a stem failure.

Basic question. "What tires do I need?"

2025-08-01T08:36:00.000-07:00

There seems to be some confusion on tire selection for RV applications. As a Tire Design Engineer with over 45 years of experience. I would like to offer some basic advice as not everyone has worked with tires or RVs.
First, you need to confirm the original Type, Size, and Load capacity as provided by the RV company. A Certification Label with GVWR (Gross Vehicle Weight Rating) and GAWR (Gross Axle Weight Rating), Tire Type, Size, Load Range, and Inflation, along with the VIN and some other info, was applied according to Federal Law to all vehicles sold. Since about 2004 Class-As have had the label near the driver's Left Elbow. It may be on the floor or the interior wall of the motorhome. Class B and Class C would have the label on the "B" pillar, AKA Door Jam. Towables had the label applied to the Outside, Driver side front 1/4 of the RV length.
Tire Type (identified by letter before the numbers in the tire size) could be "LT", such as LT235/80R16, or "ST", such as ST205/70R15, or "P", such as P235/75R15 or have no letters in front of the numbers, such as 345/80R22.5.
Tire Size is ALL the numbers such as 235/75R15 or 295/80R22.5 or 195/75R15
Load Range is a letter such as "C", "D", or "E" on Class-C RV. letters "C" & "D" are most likely found on Class B or Van type motorhomes. ST type tires on trailers will most likely have the letters C, D or E. Class-A Motorhomes probably have the letters starting with E, F or G and can go on up to H or J. Some smaller trailers may have "P" type tires with the inflation number on the tire in the 30's and up to 50 psi if they also have "XL" as part of the tire size nomenclature.

An important bit of information you need to have is a truck scale reading when the RV is fully loaded (all the water, food, clothes, toys, fuel, and other "stuff" etc, you expect to carry.) The scale reading should NEVER exceed the GAWR found on your Certification label. That weight should also be below 95% of the "Max Load" shown on the tire sidewall or found on the Certification Label.
Example if GAWR is 4,400# which is the max on that axle then the scale reading for the 2 tires on that axle should be no greater than .95X4,400 or 4,180# for the 2 tires or 2,090# for each tire. Hopefully, the tire "Max Load" number molded on the tire sidewall is greater than 2,090#. Actual load should ALWAYS be lower than the GAWR and ALWAYS be lower than MAX Load specified on the tire when fully inflated.

TPMS or Tire Pressure Monitor System. Good ones run $300 and up, depending on the number os tires you should monitor. Some folks say, "I check inflation at every fuel or rest stop. That's good to do, but I bet you drive a lot more miles down the road than you do while in a fuel stop or rest area and clearly you are more likely to get a puncture while driving than while you are stopped.

If you ask people who have had a tire failure because of a puncture, ask them what the total cost was in time and money for them to replace the punctured tire.

A question on Max inflation on the wheel

2025-07-28T07:12:00.000-07:00

Question and post from Doug, a reader of this blog

Roger Marbles' tire dissertation…. one item I constantly see miss is the allowed tire pressure on the steel wheel. Each wheel is stamped with a code from the National Wheel and Rim Assn. I just happen to have acquired one of their spec sheets. The wheels on my Class A are stamped (embossed) “AF” which means the maximum pressure is 95lbs. Is that ambient cold or maximum hot? That I don’t know and it isn’t explained. I presume it is cold or ambient. So, is there a benefit in buying a higher maximum load tire or not?- If the wheel can’t take up to, say 120lbs of pressure! (Load range F tire).

My reply: Unless you are fine-tuning a race car handling, the inflation we are talking about is ALWAYS the “Cold” inflation of the tire. RVtravel.com has a section on “Maintenance & Repair Archives.” Just click in the blue header to find any of my tire-related posts. Or you can click on this to get to my posts. You can also do a “word search” if you do not see the topic you are looking for. BUT in this case I previously covered. If you need greater load capacity, just changing the tire does not increase the GAWR spec for your RV, as the GAWR spec covers the tire, wheel, hub brakes, nuts and bolts, and axle and mounting brackets, etc. So just increasing the load capacity of the tire will not result in an increase in the max load capacity of the RV.

Hi Roger, I am well aware of the tire ambient air psi, which I have read in your posts numerous times. My specific concern is with the steel wheel vs the rising pressure when traveling down the road. If the wheel is allowed to hold 95 psi, is the rising rolling pressure increase above that safe for the steel wheel? With an F load tire, the MINIMUM pressure to support the load is – say 105 lbs cold, and a wheel at only 95 lbs cold – already over the max steel wheel psi – what happens when the pressure increases even higher to 120-130 or more psi from rolling heat buildup?

Doug, I thought I was clear but the ONLY pressure we are concerned with is the “COLD” pressure. This applies to both the tire and the wheel. In your example, something is wrong. Either the tire or wheel is too small or has too low a load rating or the load being placed on the tire & wheel is to great. If you know the actual load on the tire & wheel and the inflation shown in the tables for that size tire and wheel is greater than that specific tire capacity, then you are overloading the system and need to reduce the load and or increase the size tire or the tire Load Range (Psi). Example below:
Assume we have a 225/75R19.5 LR-E tire. That tire is rated for 3,195 @ 80 PSI cold in a Single application. Also assume the wheel is rated for 80 psi. If you go on a scale and learn the tire load is 3,445# you MUST change something. Either change to a stronger wheel & Tire or lower the load. A LR-F tire could support 3,640# BUT that would require 95 PSI rated wheels. Just changing the tire to LR-F and increasing the inflation DOES NOT increase the load capacity of the wheel, which is limited to 80 PSI cold.

Stainless air hose extender

2025-07-24T09:19:00.000-07:00

Received this question:

"What is the easiest way (tools, parts, technique, education, practices) to easily add air to the inside dually of the 1996 Holiday Rambler Navigator 40 pusher and daily work truck 2007 GMC 3500 Sierra. I have large hands and have trouble with this simple task. I have been reading and enjoying technical knowledge for some time now."

The quick answer is just to get a "double foot" Air Hose Extender end as seen here.

However, having used this type of hose end myself, I know that achieving a solid connection to a valve with minimal or no air leakage can be challenging.

Also, what you do if you are using an external TPMS sensor on the end of your valve stem? It can be difficult to get the sensor screwed off and back on again without dropping the sensor down between the inner and outer tire. My solution was to get a set of "Hose Extenders as seen here

This is a Wheel Master hose kit #8001 as seen on my RV.

I DO NOT recommend the similar "airless" hose kits from other manufacturers as your TPMS sensor will not register the tire pressure

I selected and prefer this hose kit because it provides for a SOLID attachment for the hose ends.

I see posts with complaints about TPMS sensors on the end of a hose or even just the hose itself failing and causing a loss of air but I believe it is not the hose itself that fails but the flimsy "spring" clips or rubber donuts that are suppose to support the hose where it goes through the "hand hole" openings in the wheel.

The task to install this extender kit does require some mechanical ability, a few tools and a bit of planning. You will need to do all these things safely.

Jack up the rear axle and remove the outer tire. Be sure the jack is on solid ground and using a 6 point socket of the proper size to remove the lug nuts. I keep the inner dual on the hub by placing at least 2 lug nuts back on the studs finger tight, to keep the wheel attached to the axle while working on the hose attachment
Attach the hose with the straight end to the metal valve stem after cleaning the threads and confirming the valve core has no leakage. Now is the time to replace the valve core with new core and I recommend you use the "clicker" valve core tool available at many auto parts stores. The tool is less than $8 and "clicks" at the specified 3 to 5 In-Oz torque.
Attach the hose end with the 90° end to the outer wheel (that also has new valve core) and run the hose back through the hand hole.
Orient the outer wheel with the hose at 180° from the hose from the inner wheel and re-mount the outer wheel. Be sure neither hose can contact the edge of the outer wheel hand hole. Be sure to apply the lug nuts with the proper torque spec for your RV. I have a low-cost torque wrench from Harbor Freight, as I keep this wrench in the RV so it does not get a lot of usage. The spec for my RV is 140 Ft-Lb so I would select a wrench that is capable of at least 50 Ft-Lb greater than what is required.
Now comes the hard part. You need to attach the 90° angle brackets to the center hub after making sure the long threads on the hose are well through the bracket. There are two nuts for each hose end to secure the hose in the bracket. Once the bracket is located with no part of the hose contacting the wheel you can mark and then use the "pop-Rivits" to secure the bracket to the hub. NOTE: If you have stainless "wheel simulators" you will need to be sure the hose does not contact the sharp edge of the simulator as that can "saw" through the hose and cause a loss of air. So you need to pay attention when locating the "L" bracket to the simulator.
BIT of ADVISE, when adding air to the hose, I suggest that you support the hose in the bracket with your hand as the force from the air hose could loosen the bracket.
You can now attach your TPMS sensor to the end of the hose and you should be good to go.

New Info on Michelin Agilis CrossClimate posted on the Keystone RV Forum

2025-07-04T07:59:00.000-07:00

Micheline Agilis CrossClimate versus Bridgestone Dueler Ascent

I know this is "late information for this thread" but might help someone at a later date. The Michelin Agilis Cross Climate is not a "long distance/high mileage" tire. It's more of a "snow tire" with a commercial application. The Michelin Warranty for the tire does not have a "mileage warranty" but rather a "6 year warranty based on the remaining tread depth". That "sounds pretty good" until you start reading the footnotes in the warranty brochure for the Agilis line. Footnote 2 states: "Based on a treadwear test using tires in size LT265/70R17 121/118R on 2018 Ford F250 pickup trucks, loaded to 9800 lbs, versus the following competitors. Actual on-road results may vary. Average projected mileage to wearout: Michelin Agilis CrossClimate : 24,500 miles,"

When you "do the numbers" most people buy truck tires expecting 50-60 thousand miles of tread life. For most people with a 11-15 K annual mileage, that means 5 or 6 years on a set of tires before needing to replace them. The Agilis line "Michelin projected tread life to wearout is 24,500 miles. Somewhere between 1/3 to 1/2 the typical Michelin LTX tire.

There's an old thread from a couple years ago where one member from Florida bought Agilis tires for his truck and only got around 20,000 miles before the tires were "worn out" and needed replacement.

The Agilis is a "good tire" for its intended purpose (winter tire operation in all road conditions) but is not the best choice of Michelin tires for a "true all season tire with a high mileage tread longivity"...

Just something to think about if you're looking for "tires for a truck used as a daily vehicle and to tow a travel trailer (typically summer "hot road conditions" use)...

While I have no personal knowledge on this it is reasonable to pay attention to the use of "Winter" tires in RV service.

If I upsize my tires can I lower my inflation?

2025-06-08T08:35:00.000-07:00

I Got this question.

Hi Roger. I have a tire question for you. I own a 2021 RV with a GVWR of 8596 lbs, that I purchased used in the spring of 2024. The factory 205/75R15 GY Endurance tires were manufactured in November 2019 and I believe they are due to be replaced. I am considering replacing them with GY Endurance 225/75R15 for the increased load range to give myself a wider margin of safety as the 205’s load range is basically equal with the GVWR for the camper. My question is this:
My 205’s are rated for 2150 lbs at 65 psi, which is what I’ve been running them at. According to the Load/Infl chart, I could theoretically run the 225’s at 50 psi for the same 2150 load. Obviously, I want more load capability so I would run them at a higher psi, say 55 or 60. But is it safe to run the at a lower psi since they are rated to 2830 lbs at 80 psi? I don’t need that much load capability and would prefer the softer ride of a lower pressure as long as it’s safe to do so and not be overheating the tires.
Please let me know what you think with regards to running this particular tire at less than max psi.

My Reply

I work as the "in-house Tire Expert" for RV Travel.com, so your question landed on my desk.

First, it's essential to understand what the "GVWR of 8596 lbs" means. That rating is the heaviest your RV should ever be. In fact, RVIA recommends that the tire loading should be no more than 90% of that number. Additionally, certain assumptions are made when arriving at that number, namely that the load on your tires is evenly split between the axles and that the load on each tire on an axle is evenly distributed from end to end.

RVSEF has measured the actual load on tires on tens of thousands of RVs and learned that over half of tires in RV usage are being overloaded. This data supports the recommendation that, at a minimum, every RV should get on a Truck Scale and at least learn the weight on each axle. Getting "4-Corner Weights" is not always convenient (see RVSEF) so I suggest we assume one end is supporting 53% of the axle total. Going to this effort is important as the inflation in the tables is the MINIMUM inflation for the heaviest loaded individual tire, and all tires on the trailer should have the same cold inflation.

You mention "Safety" a few times. Tire inflation safety is not an on/off switch where tires will quickly fail if you drop 1 pis below the table number and run for 10 years if you are a few psi above the minimum.

If you review my posts on my blog RVTireSafety.net, or if you do a search on "Tire Inflation" on RVTravel.com Maintenance tab you can find many posts on the topic.

The short answer to your question on new tires:

Yes, you can go up in load capacity IF you also go up in inflation and up in Load Range

I recommend you confirm the load on each axle (when the RV is fully loaded to the heaviest you ever expect to be (full water, food, tools, clothes etc) by getting on a truck scale.

Use the heavier scale number and apply the suggested 53% figure to arrive at your tire loading number.

Consult the Load & Inflation tables for your size and Load Range tire to learn the MINIMUM cold inflation that you would use for all your tires.

On my RV, I use the above and then add 5 psi to that number and make sure my inflation never drops below the Minimum by reading my TPMS display.

How and why to photograph your tire sidewall info

2025-06-06T09:17:00.000-07:00

I recently received this comment/message/question about tire sidewall info:

A material list on the tire sidewall? I’ve never seen this. It sounds like FDA package list of: flour, corn syrup. Reading black bumps on black might work in ideal lighting, but I have trouble finding the max PSI.

I replied with this:

The list of materials and the max load numbers are easily seen molded on the sidewall of all tires per Federal Regulation for the last 30 years. The US-DOT specified size, location and content are as seen in the attached examples. These pictures were taken with natural lighting and not even in direct sunlight.

To read letters molded on your tire sidewall, you might find it easier if you get out your phone aka camera and snap a couple of pictures of this important information.

I suggest you wash your tire sidewall with soap and water and snap pictures of three areas with the tire in full sunlight. Here are examples from three different tires:

1. The complete DOT serial

NOTE: You should confirm all your tires have the same serial number. If not, get a picture of each serial number.

2. The tire max load information

3. The tire material list

Note on “Max psi”: You should focus on the “Max Load” and understand that the tire is only rated for that load when the tire is inflated to the stated PSI, and the inflation is checked at ambient temperature. The “Max psi” IS NOT the highest level of inflation a tire can tolerate. Undamaged tires can tolerate much higher PSI when warm from running.

Keep in mind, if you have the tire sidewall info handy in a photo or on your phone, it will be much easier for you than if you have tire problems in the middle of the night and don’t know what tire you need for a replacement.

Can you really tow your trailer at almost 90 mph?

2025-04-26T12:10:00.000-07:00

In September 2024 I wrote about the operational limits on ST-type tires. I suggested that the published 65 mph speed limit was a realistic compromise. But apparently many people think they can get tires with some “magic” rubber in them so they can safely drive faster. Therefore, I am again writing about the “speed rating” of ST-type tires.

In the U.S., “speed rating” is really a measure of tolerance for high heat and is based on a test with speed increasing every 10 minutes until the tire fails. In Europe, Asia, and the rest of the world, they do not offer an ST-type tire because their speed tests are more realistic and stringent.

When radial ST-type tires were introduced to the U.S. market some 45 years ago, they were given a higher load capacity rating than a same-size LT tire. However, the trade-off was stated in the industry standards books as having a max operating speed of 65 mph, with load and inflation adjustments required if operating above 65 mph. With 50 years of tire design and testing under my belt, I know of no “magic” rubber that will deliver greater load capacity for a given size and inflation without some trade-off.

Five variables for operation speed

There are only five variables that control operation speed: size, load, inflation, speed and the ability to be rated for operation with passengers in the vehicle. So, unless you invent some “magic” rubber, you are limited by physics.

Yes, materials have improved over the last 45 years after the switch to radial construction, and we have seen improvements in tire performance. But have you ever asked why a company can make a tire of the same physical size and same load capacity at a given inflation and somehow make one tire carry significantly more load than the other without some trade-off in operation speed?

If I had invented some “Super Magic Rubber” for ST-type tires that gave them “Super Strength,” why wouldn’t I put the Super Rubber in the other tires I make and sell and take over the worldwide tire market? Any company can ignore reality and make a marketing decision and claim a “Speed Rating”.

Does any tire company have some "Magic Rubber:?

2025-04-08T14:44:00.000-07:00

In Sept 2024 I wrote on the Operational limits on ST type tires. I suggested that the published 65 MPH was a realistic compromise but apparently many people think they can get tires with some "Magic Rubber" in them so I am again writing about the "Speed Rating" of ST type tires.

In the US "Speed Rating" is really a measure of tolerance for high heat and is based on a 10-minute long test with speed increasing every 10 minutes till the tire fails. In Europe, Asia and the rest of the world, they do not offer an ST type tire because their speed tests are more realistic and stringent.

When ST type tires were introduced to the US market some 45 years ago, they were given a higher load capacity rating than a same-size LT tire BUT the trade-off was STATED in the industry standards books as having a max operating speed of 65 mph. With 50 years of Tire design and testing under my belt, I know of no "Magic" rubber that will deliver greater load capacity for a given size and inflation without some trade-off.

There are only 5 variables that control operation speed. Size, Load, Inflation, speed and the ability to be rated for operation with passengers in the vehicle, so unless you invent some "magic rubber" you are limited by physics. Yes, materials have improved over the last 40 years after the switch to Radial construction and we have seen improvements in tire performance, but have you ever asked why a company can make a tire of the same physical size and same load capacity at a given inflation and somehow make one tire carry significantly more load than the other without some trade-off in operation speed?

If I had invented some "Super Magic Rubber" for ST type tires that gave them "Super Strength" why wouldn't I put the Super Rubber in the other tires I make and sell and take over the worldwide tire market? Any company can ignore reality and make a marketing decision and claim a "Speed Rating".

At what temperature reading of TPMS should I worry?

2025-04-04T16:33:00.000-07:00

Read a forum post with a question

Hi all, very newbie here.

I have a 1/2 ton double axel travel trailer with ST225/75 R15E Goodyear Endurance Tires on it. I was towing it today and my TPMS monitor alarmed because of tire temperature.

Now nothing extravagant, tires were between 75 and 82 PSI, running temps for all were not above 85 degrees.

The TPMS was at factory settings. I found this on the internet, this guy says 158 degrees is a standard alarm trigger for TPMS. Is there a critical do-not-exceed tire temperature? What should I set the temperature tolerances at in the TPMS so it is not alarming with basic driving?

Here is my response

As an actual tire design engineer who has been working with TPMS since 2002 I can suggest that you not worry about the "High Temp" warning as it is normally set to 158F. You need to pay attention to the PSI. I suggest the low pressure warning be set to the minimum psi required to support your MEASURED inflation level. I suggest that you never operate the tire in overload. TPMS Temperature is not reporting the hot spot of the tire as that point is located about 1/4" to 3/8" inside the shoulder of the tire. Rubber is an insulator so it does not transmit the heat energy to the tire inside air, TPMS temperature readings are closer to the temperature of the metal wheel, hub and wheel bearings as the metal does transmit the heat of these metal parts quickly. A high Temp reading from the TPMS without a significant increase in PSI is probably warning of a wheel bearing problem.

What is the proper inflation for my tow vehicle and RV trailer tires?

2025-04-03T08:03:00.000-07:00

I monitor a number of RV forums and when I see someone asking about tires, I make the effort to learn about the problem and offer solutions, if possible.

Recently there was a question on inflation for both the tow vehicle and the travel trailer. The comments seemed to be all over the place, so I decided there was a need for some basic foundation of information. Following is my reply and attempt to provide the basic information needed by RV owners.

Tow vehicle (TV)

Tire size, type and inflation should go by the Certification Sticker on the driver door jamb. This was established by a team of tire and vehicle suspension engineers over a period of two to four years. There were numerous changes and tests run on different specification and performance parameters that were fine-tuned to meet the goals of the vehicle design team. The sticker provides a recommended inflation that normally delivers 25% to 35% Reserve Load, so those numbers should be followed unless you have done a lot of research and understand the trade-offs. Do not forget that few people in the tire store have technical training beyond how to sell tires, so I would be careful with the information they provide.

Travel trailers (TT)

Tires for almost all RV trailers have a single spec—that being Low Cost. I have never heard of any vehicle testing or evaluation that compares different tire constructions or ratings for the application of a tire to the travel trailer.

To achieve low cost, tires are only required to meet a single requirement: that they meet the minimum load capacity specification needed to support the RV. In 2017, the RV Industry Association (RVIA) established a Reserve Load of 10%, but for RVs built before 2017 there was no Reserve Load requirement.

Those of you who have read the owner's manual and reviewed the Certification Label information and the Load and Inflation information molded on the tire sidewall may have noted that for most RV applications, you can only achieve the tire's 10% Reserve Load capacity when inflated to the level required for the maximum load capacity.

Yes, this basically means you must run the max inflation for the tire's Load Range if you want to have the minimum margin of Reserve Load recommended by the RV Industry Association.

Tire type nomenclature

In the subject post there was some confusion about the Michelin "LTX" line. One poster thought this was some special "Hybrid" type of tire, so I feel the need to clarify tire "TYPE" nomenclature.

The letters before the numbers in the tire size are critical and important but too often people seem to ignore that important info. The P is for Passenger, LT is Light Truck, and ST is Special Trailer. ST should only be used on trailers and are not approved for use on motor vehicles that carry people. if a P type is applied to a trailer, its load capacity MUST be reduced by dividing the load capacity stated on the tire or in the Load & Inflation tables by 1.10. But if an LT tire is applied to a trailer no reduction in load capacity is required.

Eventually, the following information was added to the Forum thread:

------------------ Steer Axle ---Drive Axle --- Trailer
Weight #1 - Truck only/hitch/bars -- 3180 3520
Weight #2 - Truck and Trailer with WDH bars -- 2980 4320 4880

Side note: The owner had previously stated the TV GVWR was 6500, so his scale reading on the initial weighing seems to exceed his GVWR !

I am assuming the "Trailer" scale was for the four tires on the RV. If so, it sounds like the trailer was empty when he did the weighing.

In the thread, I also saw a comment about Michelin tires. The owner said:

It was helpful to learn of both the Michelin "hybrid" LTX and the Cooper XL alternatives to LT tires although when I looked up the Michelin LTX I saw that it comes in A/T and A/S versions ... even more confusion ... is the A/S the "hybrid" and the A/T the real LT??

My response

I'm not sure what you mean by "Hybrid". Michelin has a "line" of tires it calls "LTX"— this includes both passenger type and LT type tires. Think of this as with General Motors, which has a "line" of cars called Chevrolet with many type vehicles from sedans to SUV and pickups.

In my post I covered the three basic families of consumer tires: Passenger, Light Truck and Special Trailer. Each "family" has many different sizes and many different tread patterns, like the A/T, that is a traction tread pattern, and the A/S which is All Season and provides less off road or snow traction.

Load Capacity

There are also within each family different levels of Load Capacity.

P type "family" tires come in "Standard Load", that has no special markings, and also XL for Extra Load.

LT and ST "families" have greater load capacities, so they have letters assigned to the "Load Range", or LR for short, with the different LR having letters starting at C, D, E and on up in some brands. Each letter has a specific maximum load capacity at a specific inflation. The load capacity is published in tables available in some owner's manuals or tire data books, or even on the internet.

All of the above applies to tires made to U.S. Tire & Rim Association standards. Europe has some similar but different standards with different codes, as does Asia. I will not go into those standards, as I probably have already overloaded you with the above.

Clearly, the owner of the truck and trailer needs to learn more facts about tires and proper loading of his TV truck.

What type of tire should you run

2025-01-15T10:38:00.000-08:00

Ok, Lots of opinions and partially correct information here. Lets go with the data and facts.

First RV manufacturers are required by law to apply tire placards that state both the tire size/type and the MINIMUM inflation you should run. This is the same type information you will find on your Smart car or your F350 dually.

Second There are a number of "type" tires that the RV owner might run into:

P-Metric These are primarily intended for Passenger car applications. They can be used in other applications but their load capacity needs to be adjusted DOWN by 10%. P-Metric are capable of sustained 85 mph or greater. P235/75R15 105S is capable of 2028# @ 35 psi in car application. Special Note The load is reduced to 1843# at 35psi in Light Truck, Station Wagon, Van, Trailer or Bus operation. The 105 is the Load Index and the "S" is the speed rating, in this case 112mph. I do not find any information on Dual application of P-metric tires in Industry standards.

LT-metric These are primarily intended for Light Truck application and most have a RADIAL construction as in LT235/75R15 104/101R LRC. This tire is rated at a Max of 106 mph and at 50 psi can carry 1985# single or 1820# when mounted in Dual application. For reference at 35psi it can carry 1530# or 1390#. Special note on speed. If a LT tire is operated above 65 mph it's cold inflation must be increased by 10psi but never above the tire Max. If you cannot increase the inflation you Must decrease the load.

ST These are Special Trailer tires and come in a more limited range of sizes. Many ST tires are Not Radial but carry a "D" in their size indicating Diagonal or Bias construction. An ST225/75R15 is rated at a Max of 65 mph and 2150# in Single application at 50 psi minimum. I do not find any information on Dual application of ST tires in Industry standards.

High Flotation tires such as 31x10.5R15LT 109R LRC are sometimes considered for the pickup tow vehicle. This tire is rated at a Max of 106 mph and at 50 psi can carry 2270#. at 50 psi. It is 30.52"OD while the LT235/75R15 is listed at 28.9" OD. I do not find any information on Dual application of High Flotation tires in Industry standards.

Class-A RV TBR (Truck Bus Radial) such as 295/80R22/5 is rated at 7830/6940 Lbs Single/Dual at 125 psi. For this application it is imperative that you check the tire manufacturer's data sheets for the details on your specific application and size.

Third and most important - ALL RV applications. You should visit http://www.tiresafety.com and look at the tire RV specific info under Size & Classifications on how to properly weigh your FULLY LOADED rig and how to calculate the individual tire loads. Only with this information can you really know what Minimum size and inflation you MUST run.

Do not do this to your RV tires

2024-12-29T13:28:00.000-08:00

Do not bleed down your RV tire pressure because you think it will “explode” if you exceed the PSI number molded on the tire sidewall.

Almost every week I see posts on one or more of the various RV forums where people say they limit the PSI in their tires. Sometimes they say why, as in “to prevent a blowout.” Sometimes they do not say why, but their reasoning behind this practice is clear to me.

It appears that many people incorrectly believe that exceeding the “PSI” number molded on the tire sidewall is going to result in tire failure and explosion.

It may help to let people know about some of the testing that tires undergo during the development phase.

First some background

The “Max Inflation” statement on your tire sidewall really is the inflation associated with the Maximum Load capacity. If you actually read your tire sidewall, you will see that is what it says. So the number of PSI is really the minimum needed to support the stated load.

It is important to understand that we are always talking about the “cold” inflation and not the inflation of a tire that is running down the highway or has been in direct sunlight or driven on in the prior couple of hours. “Cold” inflation does not mean the tire needs to be refrigerated. It means the tire is at the prevailing ambient (surrounding) temperature.

Tire pressure monitoring systems

With the introduction of aftermarket tire pressure monitoring systems (TPMS) that report both tire pressure and the temperature of the sensor, people are now being exposed to information they have no previous experience with. They see tire pressure rising as they drive down the highway. Some may see a 10% to 20% rise in pressure from the pressure they set their tires to just a half-hour earlier. This increase is a cause for concern, not because they had any working knowledge of tire operating temperature or pressure, but simply because the pressure was higher than they expected.

What not to do

A few people have decided that they need to bleed down the high pressure because they thought the pressure number of the tire sidewall was the maximum it could tolerate. Of course, the action of bleeding down the hot pressure was exactly the wrong thing to do, as that meant that the tire would no longer be operating at the pressures expected by the tire engineer when they originally designed and tested the tire specification.

Obviously, this raises the question of how much pressure a tire can tolerate before the owner should be concerned. Well, I am going to give you some numbers, but these are only examples. I cannot speak for every tire company’s process or specifications. But if we start with a few guidelines, I think you can get comfortable and, hopefully, you will believe that tire design engineers do have a reasonable idea of what they are doing.

First off, we are only going to discuss regular production street tires that have passed DOT testing. The numbers I will use would basically be seen on new tires. I cannot advise on a tire’s capability after it has been damaged or run for tens of thousands of miles. Damaged tires can fail when reinflated, as seen in this video of a 22.5-size tire.

This video was made to show that welding on a wheel with a tire still mounted can result in a tire failure. Note the pressure at failure.

Tell service person if you drove on under-inflated tire

If your tire suffered a puncture and you drove any distance with the tire underinflated, you may have permanently damaged the body cords. That is why you should always tell the tire service person that you drove on the under-inflated tire so they know to use a cage or other restraining safety equipment. Age and miles can reduce the strength of any tire, but we do consider this degradation when approving a design.

Another consideration is the availability of high-pressure air. Most home compressors or those that supply air to hoses made available to consumers have an upper limit of about 150 PSI. Therefore, we don’t expect any consumer to have access to or to use industrial air pressure, which can be 400 PSI, or even special inflation units as used on aircraft tires.

Basically, many tires are tested to 300% to 400% of their rated inflation, or above 200 PSI, whichever is lower. I had a number of tires that could contain over 300 PSI when new. If they had been damaged, however, the same tire might fail at less than 200 PSI. But I also never, in my 40 years as a design engineer, saw a tire fail from normal pressure increase—and I was involved in the testing and evaluation of thousands of tires.

The bottom line

The bottom line is: Please do not bleed down your hot inflation pressure. You should ONLY set and adjust inflation when the tire is cool and at ambient temperature. Doing otherwise MAY result in you having a failure days or weeks later because you were running lower pressure than what was needed for the tire load you were applying.

Also, unless you have damaged your tire, it is unlikely to “explode” simply by adding 10% more cold air pressure.

Tire Inflation. We all need to pay attention to it

2024-11-29T09:00:00.004-08:00

It doesn't matter if we drive a Diesel Pusher that is 48' long or pull a 6' "Pop-Up" trailer with our car. It doesn't matter if our vehicle runs on Gas or Hydrogen or Diesel or is an EV, our vehicles currently run on tires that are inflated with "air". Even if that inflation "air" is 78% Nitrogen or 98% Argon we must use some gas in our tires. What is constant for all vehicles on the roads today is that we have tires that require inflation with some sort of "gas".

It is basic Physics that It's the "air" pressure that supports the tire load, not the tire structure, and while we all know that there are folks on the Internet, who will claim to know more about any topic than the people with years of training, I can only hope that the majority of readers of this blog will trust the actual Science of tire design and engineering rather than some random guy spouting his opinion on what level of inflation you should run.

I follow a number of RV forums and find a number of folks offering their interpretation on what the proper inflation is for tires in RV application. Some insisted the only correct inflation pressure was what was shown on the "Tire Sidewall" while others insist the "Vehicle Certification label" is the only correct level and of course there are more than a couple that have developed their own inflation tables and will be happy to tell you what their personal calculations indicate.

Here is my take on the subject:

We have two separate issues to consider when selecting the minimum cold tire inflation pressure in RV application.

A vehicle MFG has DOT requirements they must follow. They are to specify the minimum inflation needed in the tires they provide on the new RV. That inflation is required to be sufficient to support the stated GAWR divided by the number of tires on that axle. Basically this means the axle rating is used to establish tire inflation based on the published load & inflation tables from the tire industry.

The owner of the RV needs to confirm the actual loading for their RV and should confirm they are not exceeding the GVWR or GAWR or the individual tire load capability.

You will immediately note that the RV company is making an assumption that is just not a reflection of reality, that being that axle loads are always split exactly 50/50 side to side. While many RVs may have the load split only a couple percent off of 50/50 some have discovered one end of an axle may carry more than 1,000# more than the other. Others have learned their unbalance is closer to 40/60 and some have discovered that their tires are overloaded as soon as they fill their water and fuel tank before loading anything else into the RV. Obviously a responsible owner needs to do a better job than some RV companies are doing.

While some feel it is sufficient to just read the regulations about the vehicle certification label, I and other tire engineers know that "policy" is not always the best way to get the best combination of ride, handling and tire durability.

Lets see what Goodyear says in their RV tire application web page.
"Correct tire inflation is a key component in tire care. The recommended inflation pressures for your tires are indicated on the certification label or in your owner's manual. Since RVs can be loaded with many different configurations, the load on each tire will vary. For this reason, actual air pressure required should be determined based on the measured load on each individual tire. Inflation pressure should be adjusted to handle the tire carrying the heaviest load, and all tires on the axle should be adjusted to this standard."

Maybe you don't think Goodyear knows more than some RV owners. What does Michelin say in their RV documents Here on pg 4:
"The amount of inflation pressure required in each tire depends on the weight of the fully loaded vehicle, to include passengers, cargo load, fuel, and water."
" To determine proper inflation pressure, Michelin recommends weighing each wheel position of the recreational vehicle individually. Weighing each axle end individually will give a clear indication of how the weight of the recreational vehicle is distributed across the axle. "

Also in their tire maintenance document on tire inflation:
"Unlike commercial light truck and truck applications, we recommend weighing motorhomes at each wheel position of the vehicle (right front corner, left front corner, etc.). This is the only way to get a clear indication of exactly how the weight of the motorhome is distributed. Otherwise, one wheel position may be significantly overloaded even though the GAWR has not been exceeded. Tire inflation for a given axle should be set according to the pressure corresponding to the higher of the two end loads."

Others are allowed to have and express their opinions and their interpretation of how the regulations are to be applied, but it was my job as a tire engineer to understand and work within FMVSS regulations for some 40 years.

I will leave it up to you to decide if the only acceptable inflation is what is on the vehicle certification label or that inflation is learned by reviewing what major tire companies say in their published guides or if some guy pontificating at the campfire is a more reliable source.

Maximum Operational Tire Speed

2024-09-02T17:59:00.000-07:00

There are still a number of folks asking about tire operational speed and specifically with ST type tires. I hope this post will help.

Back in the 70's there was a National Motor Vehicle Speed limit of 55 MPH, and the RV market wasn't as big as it is today. Plus RV's were much smaller vehicles and many used Passenger Vehicle type tires.

Then the RV market started to grow and RVs became larger and heavier so tires offered on RVs needed to increase in capacity.

Based of info from someone in the tire industry at the time, we know that some RV companies did not want to use P type tires because of the loss of 10% of the load capacity and they did not wan to.pay for more expensive LT type tires so the RV companies asked for "Special Trailer" tires.

Not all tire companies agreed to the proposal and even today many companies do not make ST type tires.

The formula to calculate tire load was adjusted and included a statement on 65 MPH max speed. This showed up in the publish Industry standards as seen here.

Goodyear even issues a Bulletin PSB #2011-13 in 2011 stating that "Industry standards dictate that tires with the ST designation are speed rated at 65 MPH (104 km/h) under normal inflation and load conditions." Adjustments for higher speed up to 75 MPH would be allowed if there were either a decrease in Load capacity or increase in Inflation limit or both. The wording is convoluted but it specifies "Based on these industry standards, if tires with the ST designation are used at speeds between 66 and 75 mph (106 km/h and 121 km/h), it is necessary to increase the cold inflation pressure by 10 psi (69 kPa) above the recommended pressure for the rated maximum load.
- Increasing the inflation pressure by 10 psi (69 kPa) does not provide any additional load carrying capacity.
- Do not exceed the maximum pressure for the wheel.
- If the maximum pressure for the wheel prohibits the increase of air pressure, then the maximum speed must be restricted to 65 mph (104 km/h).
- The cold inflation pressure must not exceed 10 psi (69 kPa) beyond the inflation specified for the maximum load of the tire."

Please note that the Goodyear bulletin does not completely match the publisher TRA guidelines.

In 2017 the US federal Trade Commission placed a tariff on all "non-speed rated" tires so the tire companies in Asia where the low cost ST tires were made quickly adopted the use of a "Speed Symbol" based on the use of the SAE Passenger car Speed test to justify the use of higher speed rating.

This "Speed Test" only requires that brand new tires be capable of running 10 minutes at a stated speed so in reality the SAE test is just a high temperature resistance test. After running the test, tires are not subjected to any additional testing and are considered scrap.

Through all of this there has been no adjustment to the original load calculations that specified a 65 MPH Maximum operational speed.

We have all learned or read of the general degradation of tires in Trailer application with Interply Shear being a major physical cause for this degradation of the belt strength which can eventually lead to a "Belt Detachment" AKA "Blowout".

So what does this all mean? In my expert opinion:

1. ST type tires should not be run for any length of time above 65 MPH at the published Max Load for the given Load Range of the subject tire unless adjustments to load and inflation are made.

2. If you want to run your ST type tire at a speeds above 65 but no greater than 70 MPH you MUST increase the cold inflation by 10PSI. This inflation increase DOES NOT provide any increase in load capacity

3. If you want to run your ST type tires at speeds above 70 but not greater than 75 MPH you MUST increase the cold inflation pressure by 10 psi PLUS you must reduce the tire load MAX capacity by 10%

4. At no time should you ever increase the cold inflation pressure above the maximum PSI rating of the wheel as identified by the wheel manufacturer.

You should note that the Loads and inflations published in the Industry Standards book AKA TRA manual are based on a maximum operation speed of 65 MPH. The "Speed Symbol" is a sales and marketing tool and I find no information in the TRA manual that recognizes the SAE speed Symbol as superseding the existing TRA standards.

If or when such approval is issued by TRA I will adjust my statements above accordingly.

If you want to understand Interply Shear I suggest you review the following posts.

- https://www.rvtiresafety.net/2013/11/interply-shear-and-other-techno-babble.html

- https://www.rvtiresafety.net/2018/09/warning-super-technical-post-tire.html

- https://www.rvtiresafety.net/2018/07/question-on-radial-tire-belt-interply.html

- This link has a video showing the effects of tire cornering and the resultant Interply Shear

https://www.rvtiresafety.net/2018/02/tire-inflation-not-same-for-all-trailers.html

I hope this clarifies the question of why ST tires in RV service seem to fail at a higher rate than regular Radials when used on cars and Light Trucks.

Reader's questions on inflation and type tire load capacity.

2024-08-24T06:55:00.000-07:00

Questions from Readers

"Is there a way to verify the tires on my trailer have been inflated with nitrogen? My tire dealer said they did but I am not sure if I can trust them. The pressures stay even tire to tire and not a lot of change while driving which I think tells me that the tires are inflated with nitrogen."

My answer: As a Tire Design Engineer, I see no reason to worry about the claimed use of Nitrogen, The air we breathe is 78% Nitrogen. While using N2 to inflate tires is not bad it just isn't worth spending any money on it.

About your question on inflation: Are you using your TPMS to monitor tire pressure or are you using a hand gauge? TPMS are generally accurate to +/- 2% and they can warn of puncture while driving down the road. I have seen a tire fail because the owner used his hand gauge but the valve core stuck slightly open and the tire failed 50 miles down the road.

Here is a post of my inspection of that tire. I believe that if he had been running a TPMS he would have avoided that failure.
https://www.rvtiresafety.net/2020/11/claims-of-defective-tires-causing.html

+++++++++++++++++++
A question on "LT 225/75/16E. vs. 225/75/16 Commercial tires"

"I’m looking to replace tires on E-450 chassis motorhome. The OEM tire is a LT 225/75/16E. Reading the forums, a number of people are replacing the OEM tires with 225/75/16 Commercial rated tires. They are doing this because the OEM Load Capacity is very close to the sum of the load of the four E rate tires. Four Commercial adds appx. 500 lbs. per tire load capacity or 2000 lbs. for four tires."

My research has turned up seven tires that have both an E rated and Commercial in the same model tire. What I found and surprised me was the as the weight difference between the two tires. With the exception of one brand, all show that the Commercial rated tire weighed less than the LT E rated tire. As a lay person, it seems counter intuitive to me. While there are other tires, here are two tires I am considering:

Michelin Ailis CrossClimate
Commercial: 37 lbs.
LT E rated: 39 lbs.

Toyo Celsius Cargo
Commercial: 36 lbs.
LT E rated: 38 lbs.

I would have thought the commercial tire would have been heavier given that the commercial tire has appx. 18% more load capacity. Can you explain and shed some light on this?

Thanks

My answer:

ED We need to be careful when comparing tires as the actual "TYPE" and Load Range can result in our selection.The Michelin Agilis is a good example. This is a tire developed in Europe and does not carry the "LT" designation as in Europe they call some lines "Commercial" and other tires of the same size are not considered "Commercial". The Load Index number is the tip off. You can consider this a bit like the "Load Range" AKA "Ply rating" in the US market.
The additional load capacity differences come about because of the conversion of Metric to "English" dimensions. I think that if you look at the actual specifications for the tires you are looking at, you will find the stated inflation numbers are slightly different. Close but different. The expected usage can also result in some tires being considered "Commercial" vs "RV".
I do see many people asking about or posting information on tires but they do not include the "Complete" designation in their Internet posts. Some include the ST or the LT on US sizes. European tire sizes do not have "ST" type tires or "LT" as leading letters but have a "C" after the rim diameter standing for "Commercial". (Not to be confused with Load Range C) To further confuse the issue some tires are available with two different Maximum Inflations and different Maximum load capacities.
For your question I would suggest you look at the Maximum Load Capacity and the inflation necessary to support that load. If the inflation is higher than that shown on your Certification Label, which I think might show an LT type tire in Load Range E and 80 psi. Your "Commercial grade" European tire may specify inflation a bit higher than 80 psi. If so you need to confirm the max inflation rating for the new tires.

I would not be concerned with the weight of the tires you are considering as design and wear goals can add or subtract a couple of pounds per tire
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I got this request:

How about a recommendation for a quality tire pressure gauge? I no longer trust the typical stick gauge.

I have found the Accutire MS-4021R Digital Tire Pressure Gauge from Amazon to be very accurate.

I have 3 and they always read +/- 0.5 psi from the ISO Laboratory Master I used to check my gauges.

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This question from a Class-A owner:

We have a Class A diesel MH. Our maximum cold pressure on the front tires is 120 psi. After weighing our coach we need to run 113 psi. We add an extra 5 psi for safety, so our cold pressure is 118 psi.

We live along the Texas coast, so our summer morning temps can be in the high 70's or low 80's. Before leaving for a trip I set my cold pressure to 118 psi first thing in the mornings.

During the summer we often travel to Colorado or the mountains of New Mexico, chasing the cooler temperatures. A few weeks ago while in New Mexico the morning temperature was in the high 40's and my tire pressure had dropped to 106. We were leaving that morning for Amarillo TX. The high that day in Amarillo was to be 101. My dilemma: add air to increase my pressure to 118 and chance once getting to Amarillo of having my pressures into the high 130's or low 140's; or drive several miles on the lower pressure tires knowing that they would increase in pressure in several miles to a safe traveling pressure.

What I did was to add about 5 psi to each of my front tires. Upon reaching Amarillo my pressures were in the mid to high 130's. The next morning before starting our day I checked the pressures and released about 5 psi in each front tire to get me to my normal pressures of 118 psi.

My answer:

Yes, you set the COLD Inflation Pressure in the morning before you start to travel and before the tires have warmed up from travel or being in the direct Sun. You do not have to worry about the increase as you travel into warmer parts of the country or as the tire warms up from travel.
You never want to travel on underinflated or low-pressure tires. Your 130 or 140 psi is not a problem.
You can adjust to the cooler temperature and lower PSI the next morning after the tires have cooled down.

Your +5 psi is a good idea. That is about what I use on my RV.

Side question: Where did you get the suggestion to inflate to 113 psi cold? The Load Inflation tables are usually arranged in increments of 5 or 10 psi. You should not interpret the inflation between the individual boxes found in the tables. If you need more then jump up to the next level.

Roger Marble

*****

##RVT1173

What inflation should I run in my tires?

2024-07-15T11:36:00.000-07:00

After I published information on what "MAX" means on a number of RV Forums, I had a few folks thank me for the clarification. But they followed up by asking where to find the Inflation Tables. This request surprised me as I thought that most people knew that they can check my RVTireSafety net blog or simply "Google" about almost any topic if they had a question but I apparently was wrong.

So here is a collection of links to the various tables published by different tire companies. Since not all tire companies make all sizes you may need to check the table for a different tire company if you can't find the info for your particular tire.

Almost all tire companies follow similar load formula calculations in the USA, Europe and Asia. BUT minor differences occasionally occur due to doing calculations in inches and pounds vs millimeters and kilograms as there is rounding that takes place along with some minor differences in some "K" factors. Also some sizes have been around for decades so their load capacities may not match the formulas exactly but are just accepted as acceptably close. So there are minor variations. This is why it is best to use the tables published by the company that makes your brand of tire.

Sometimes this is easy because you use one of the big name companies such as Goodyear or Michelin while other times you may find you have tires that are not actually made by the company with the name on the sidewall but are in fact just imports made in common molds with changeable nameplates.

If you find an error or omission in this list, please drop me an email with the link that needs correcting or new link and I will update this post. That way you can bookmark this page and save it for future use.

To the best of my knowledge Both Goodyear and Bridgestone/Firestone follow the Tire & Rim Association (US) published tables so you can simply look at their tables for the numbers if you can't find your tires in any of the links below.
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If you find a broken or bad link please send me an email so I can fix it. Thanks.

Bridgestone\Firestone
Links to Load Inflation charts, How to Weigh your RV worksheet and related material. Note Bridgestone "Medium and Light Truck data Book includes Load inflation tables for 16" and larger tires for both Bridgestone and Firestone brand tires.

Carslile Has product info and size list but no Load Inflation tables However since they only have ST type tires which I suggest always be run at the inflation pressure molded on the tire you can use the max load info to confirm you are running under the tire max by at least 15% for longer tire life while still maintaining the tire max inflation. They do have three larger size ST type tires found HERE

Continental
Databook for commercial tires

Cooper Does not publish Load & Inflation tables. I suggest you use Bridgestone or Goodyear tables

Double Coin truck tire
Load & inflation for 19.5" and larger

General truck tires
Load Inflation tables

Goodyear
RV tire information including link to Load & Inflation tables

Hankook Medium-duty Truck tire info. The link will download their catalog with Load & Inflation info.

Hercules Does not publish Load & Inflation tables. I suggest you use Bridgestone or Goodyear tables. BUT their high load ST tire info can be found HERE

Kumho
Product info page and links to their technical information.
Note they show both P-metric and "Euro-metric tires that do not have a P prefix but based on the inflation these are basically passenger type tires even though some are listed under "Light Truck". Only their "Road Venture has actual LT type tires listed. They do have 19.5 and 22.5 size tires.

MAXXIS LT tires
Tire data on LT tires but no Load Inflation tables. I suggest you use Bridgestone or Goodyear tables

MAXXIS Trailer tires
Tire data on ST tires but no Load Inflation tables. I suggest you use Goodyear tables

MAXXIS Trailer Load & Inflation tables

Michelin
RV Tire guide and links to Reference Materials and Load Inflation tables for 16" to 24.5" tires in RV application. NOTE many of the Michelin tables are not based on single tire loads but axle loads so you will need to divide the Michelin numbers by 2 for Fronts and by 4 for dual position individual tire loading. This just adds a bit of confusion to your calculations. I also am aware that a number of Michelin tires do not follow the US TRA tables so if you are using Michelin brand you really do need to use their tables. 16" and other size Load Inflation tables can be found HERE

Omni-Trail
This importer is offering 75 mph speed rating on these ST type tires.

Sailun S637 17.5 size Tire information Load & Inflation tables. Here is info for their ST637T ST type tire

Sumitomo you can download their booklet on Medium Truck size tires for 19.5 & 22.5 sizes
Load Inflation table on pg 4

Towmax does not publish tables only the tire max load.

TOYO
RV tire care and Load & Inflation tables. Also a good overview of info on tires including Passenger tires. Truck size (22.5) info can be found HERE

Trailer King has tire spec sheet shat show the max load and required inflation HERE.

Uniroyal LT size info. They say to use the Michelin Load & Inflation tables. "It's important that you get all the safety-related materials that come with the purchase of new Uniroyal passenger and light truck tires. If you did not receive a warranty book, you can download one at www.uniroyaltires.com/care. If you did not register your tires, please take a moment to do so at www.uniroyaltires.com. Registering your tires is easy and takes just a minute."

Yokohama
Info on their commercial tires on pg 23 HERE.

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Others will be added when the tire company responds to my request for a link, or when someone lets me know about a link.

NOTE If you can't find your specific tire size load & infl info from your mfg you should call their Customer Service number. I also note that the Toyo technical guide has LOTS of useful information.

General information

How to read a tire Load Inflation table
Barry's Tire Tech

National Highway Traffic Safety Administration
Tire Safety

File a complaint about a tire failure with NHTSA

Air Pressure, Temperature variation
Tire Rack
or go HERE

Passenger tire inflation w/video

The Tire & Rim Association. Where to buy Industry Standards books

MasterCraft tire Education

Rubber Manufacturers Association
Links to Tire Safety information

RV Safety Education Foundation
RV education and tire weighing service

BigRig
Truck Scales

CAT Scales
Truck Scales

FifthWheel
General information and weight calculations for trailer owners

Improper trailer brake setting

2024-07-09T07:34:00.000-07:00

Just a quick post with the "Evidence" you might need to confirm improper brake force setting on a trailer.

This is definitely "Brake Flatspotting" from trailer tire lock-up. Higher force on relatively light loaded tires can cause this. Once there is a spot the next time you hit the brakes the tire is more likely to stop at the same spot again.

Here are some examples removed from passenger cars. ( note your ST type and LT type tires will look similar)

It might be a good idea to check your brake settings every couple thousand miles such as when you re-pack your wheel bearings.

Did you take a close look at the tread surface when you did your last 2,000-mile tire "Free Spin" inspection or when you re-packed your wheel bearings on your trailer?

RV tire types—Part 2: Changing from ST- to LT-type tires

2024-06-21T14:06:00.000-07:00

In Part 1, ST-type versus LT-type tires: Which are “best” for RVs?, we left off with having to do some calculations.

I will assume you have confirmed the actual individual tire loading and have moved some heavy items around to end up with a reasonable balance of loads.

I will also assume you still want to change from ST-type to LT-type tires. That means you must increase the Load Range and/or increase the tire size to get a load capacity in the LT to match or exceed the capacity of the original equipment (OE) ST-type tires.

Load & Inflation tables

If you compare the Load & Inflation tables for ST-type and LT-type in the tire Data books (ST here or LT here), you will see that there are no LT tires of the same size or Load Range as an ST-type that can support the same load. You can confirm this by also noting the “Load Index” which is a pair of numbers like this:

The dimensions are obviously 225/75R16 and the Load Index numbers are 115 for single and 112 for dual application. The “R” is the “Speed Rating” for this tire which, for RV tires, is a relative rating of the heat resistance of the tire. Tires in RV application are limited to 75 mph max, except for ST-type tires, which have a max of 65 mph, based on the Load Formula limitations. (I will cover speed in another post.)

Before we move on, you need to realize that “LT” is a designation used here in the U.S. In European and some Asian countries they have what they call “Commercial” tires. These Commercial sizes do not start with LT or CO but will probably look like 7.00R15, or for metric sizes, 205/75R16C. The “C” in this case is not the Load Range but stands for “Commercial”. The Load Range will be identified as normal LR-C, LR-D, etc., or possibly with R or XL for Reinforced or Extra Load. To make this post easier to read, I will limit my comments to LT-type tires. Just remember there are other options that may be better for those with 15″ or 14″ wheels that do not want to change rims.

NOTE: All of these letters and numbers are important when selecting a size, so be sure you record them all when doing your research.

So, on to the next step:

Dimensions

There are two key dimensions: Outside Diameter, or OD, and Width. I am confident that we all understand OD, but width can be a bit confusing. Depending on the wheel well contour, the overall maximum width or “Section Width” may be most important. Some tires may have a narrower clearance nearer the tread, so they will need some actual measurements at a number of locations.

It may be easiest to use the dimensions for OD and “Section Width” published for your current tires and just do a confirmation with your tape measure. Remember tire “width” is not the same as tread width.

You need to be sure the tires NEVER contact any portion of the RV frame wheel well or bodywork. This is especially true for the front position of a motor-home but since we are focused on ST to LT, there should be no ST tires ever placed on the steering axle of an RV. You should try to have equal or greater clearance with the new tires than you have on your original size, if at all possible.

The challenge

When moving from ST-type to LT-type, you will need to move up in Load Range or up in Size, or both.

Now comes the research to see what your options are

Knowing the target Load Capacity and the maximum OD and Section Width, it’s time to use the Internet to do some research. The objective is to find tires that meet your needs for the numbers and that have an appropriate tread pattern. You certainly don’t need Snow Tires or heavy traction tread pattern for normal RV use. I would suggest that the tread be identified for “All Position” or Steer for your trailer application. I run “HT” type tires which stands for Highway Traction.

You can go to websites from large dealers such as Tire Rack, Pep Boys, Walmart, NTB, Discount Tire, or similar. You might also just Google “Trailer Tire” + the name of a large city or town near your location. Once on their website, find the various possible tires that meet your needs.

Next, confirm inflation

If you are increasing the Load Range with the associated increase in inflation, you need to confirm the wheel can manage that higher inflation. The info may be marked on the back side of the wheel, or you may need to contact the wheel seller or manufacturer. Or you may need to get different wheels if your original equipment seller doesn’t know what the rating is for the OE wheels.

Consider tire warranty

Finally, as I have previously suggested in any comments on “The Best Trailer Tire,” you need to make your purchase decision not just on lowest price, but you also need to consider the tire warranty, if there is one. Or check if there is a Road Hazard Warranty. Also, look into how easy it will be to get a replacement if your tire gets a sidewall cut or un-repairable puncture.

Last step

After all this work, we want to do a first class job. Some might want to say you can’t change from ST-type to LT-type due to Federal Regulations. Well, that just isn’t true. The Certification Label on your RV provided some MINIMUM STANDARDS for Load Capacity and Inflation. There is nothing wrong with increasing the Load Capacity of your tires.

I hope these two posts have helped you understand the steps, calculations, measurements and research needed to make such a change.

So what speed do you want to drive with your trailer?

2024-06-13T14:40:00.000-07:00

I see many folks claiming their ST type tires have "speed Rating well over 75 mph.

They are basing this on the "heat Resistance" test that runs tires at 88% of the max load for 10 minutes steps up to the "Goal" speed. If a tire can run 10 minutes at that speed the tire is said to be "speed rated" for that speed.

I do not recall seeing any advertisement for ST type tires that mention speed mentioning "at what load"

Here is the info directly from US Tire & Rim Association.

Notice the adjustments for inflation and load used to allow a tire to run faster than 65 mph.