“How much air pressure is too much in RV tires?” is a simple question with a complex answer.
An incorrect reading of the “MAX PSI” info on the tire sidewall does not improve the understanding of this question. If you didn’t read my post on what “MAX PSI” really means, I suggest you read that post now.
With the introduction of the tire pressure monitoring system (TPMS), many RV owners are presented with what, to them, is new information on the status of their tires.
As with almost all “simple questions” about tires, the answer is not as simple as people would like.
Question I received on tire pressure
Here is an example of a question I received on tire pressure:
“I have a serious concern with the G-rated tires on my 5th wheel trailer. When I run the cold max pressure at 110 PSI, I get TPMS readings up to 134 PSI rolling down the road. Lately, I have been running 95 PSI cold and am getting 115-119 PSI rolling. The tires and rims are rated for 110 PSI. I can live with 120 PSI, but the 134 PSI concerns me.”
My reply…
The inflation ratings for wheels are based on a “cold” pressure. An increase in pressure due to operation is taken into consideration by wheel manufacturers. I would not be concerned as long as the proper cold inflation is used along with appropriate limits on load and speed.
The wording “max pressure” can be confusing. In reality, the pressure stated on the tire sidewall is the cold pressure needed to support the stated load. That load is the maximum load the tire should ever be subjected to. I advised the owner that he should NOT under-inflate his tires and plan on seeing the operation temperature result in an increase in the pressure sufficient to support the load. The “cold” pressure is the only pressure you should be concerned with.
Tire pressure increases with an increase in temperature
Tire pressure increases by about 2% for each increase in temperature of 10° F. If you don’t remember the science from high school, you can read this post.
Please note that you will see many folks saying the tire inflation will increase by 1% or 1 PSI, but those numbers are based on passenger tire levels of inflation.
If you are seeing a 21% increase in pressure (110 > 134 psi), that means you are seeing about a 100° F increase in internal tire temperature. I would consider that level of increase too much for normal tire operation. In my 50 years of designing, testing, and working with tires, I only remember measuring a temperature rise of 100° F in Indy race car applications.
If you are seeing a 25% increase in pressure (95 > 119 psi), then you are getting a 125° F increase in temperature. This indicates you are working the tires very hard and/or you probably have “wet air” in your tires. This extra “work” that is generating a greater temperature increase is not good for long-term tire life. You are “aging” the tire rubber faster. Some might want to review the “Key Point” of tire life and how increased temperature can shorten life as covered in this post.
Are you overloading your tires and/or driving too fast?
Your temperature increase indicates you may be overloading your tires or driving faster than desirable for your tire load. It is recommended in tire company data books that your operating speed for any tire in RV application to never be greater than 75 mph. The “Speed Rating” is just a short-term “heat resistance” rating and should never be considered acceptable in day-to-day RV applications.
You need to confirm that your gauge is giving an accurate reading by comparing it to a known accurate gauge.
Further correspondence with the concerned RV owner
The RV owner concerned with the temperature and pressure increase later said:
“Today I drove 250 miles and my 95 PSI tires were running 115-119 PSI and the tire temps were at 20F above outside ambient temp of 50F and 71F tire readings. I still contend that 134 PSI is way too dangerous for tires to run on 110-rated tires.”
So, I responded:
Few people realize that the pressure increase as a function of temperature is based on well-established, confirmed physics. A TPMS is not reading the actual tire temperature but is actually reading the temperature of the brass valve stem and the metal base of the TPMS itself, which is being cooled by outside air.
Air is a good insulator
Air is a very good insulator. If you think about it, you have a small column of air running up the inside of the valve stem, which makes it difficult for the heat to travel up the center of the stem and past the valve core itself. This is all while the valve is moving rapidly around and being cooled by the outside air.
I am aware of laboratory tests that go against what “common sense” might indicate. These tests confirm that the air inside a tire is not uniform in temperature, but it is always cooler than the hot spots of a tire. It is the hot spots that can result in a tire failing if hot enough for a long enough period of time.
Confirm scale readings for each axle
I have no doubt that the TPMS was indicating only 20° F above the cool 50° F outside air temperature. You should not be concerned about the hot pressure of 134 psi on tires that have a cold pressure rating of 110 psi for the max load capacity rating. That is, as long as you have confirmed the actual load you are placing on your tires is no greater than 90% of the load capacity shown in the Load & Inflation tables for your tire. By “confirmed,” I mean with actual scale readings for each axle when the RV is fully loaded.
As a tire design engineer with 50 years’ experience, I trust the science of the “Gas Laws” and the knowledge that air is an insulator and metal conducts heat from a hot source to a cooler one.
I don’t know what to advise other than to lower the operating load and speed and confirm your hand gauge is accurate. Also, always inflate the tire when cold to the psi shown in the tables for your tires at your actual loading, as continued operation at current load and speeds will certainly result in pressure readings that are well above the cold pressure of 110 psi.
Roger Marble
MORE POSTS FROM ROGER ON TIRE PSI
- Is high PSI in tires bad?
- Do your RV tires say ‘MAX PSI 80 psi’ or something similar? Don’t take it literally
- I don’t calculate my tires’ PSI daily. Why do you?
- My RV tire pressure is showing a significant increase. Is this OK?
- Is the tire pressure too high?
- Should you adjust tire pressure when temps get colder?
- Do not adjust your tire pressure for ambient temperature
- Understanding what temperature change does to tire pressure
- A quick note on tire “MAX PSI”
- If I upsize my RV’s tires, can I lower their inflation?
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If you have tire questions, check out Roger’s posts on RVtravel.com. There are hundreds of posts covering everything to do with tires.
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