By Roger Marble
Your tire says “Max xx PSI at Max Load of yyy Pounds”. Will your tire blow up if you go above xx PSI?
The simple answer is No. Your undamaged tires are not going to suffer a blowout or blow up or explode if you see a pressure greater than xx PSI on your TPMS or on your hand gauge.
There is a lot of confusion out there because people do not understand the reason for the confusing wording that is mandated by DOT.
There are Federal regulations on the words and information that must be molded on the tire sidewall. This wording has been around for years, with some unchanged since the 1960s.
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What RV owners think
We recently surveyed RV owners about what they think the tire inflation number on the tire sidewall indicates. The results show that 19% think the inflation number molded on a tire sidewall is the absolute highest a tire should ever have in it. Another 18% think that the inflation number is “the best” inflation for the tire. And 2% think it’s the lowest pressure the tire should ever have. I am very disappointed with this level of confusion.
Here is the reality
Each type and size tire and load range has a stated Maximum load it should ever be subjected to. The number is molded on the tire sidewall in both pounds and kilograms. The tire industry has published tables that provide the MINIMUM inflation a given tire needs to support a stated load. The tables clearly state that the inflation number is the inflation measured before the tire is driven or warmed by direct sunlight. This is called “Cold Inflation”. It’s not “refrigerated” inflation and it’s not some laboratory 68 F or 70 F “standard.” It is the inflation that would be the same as the surrounding ambient air. Some people know this as the “temperature in the shade.”
The confusion comes about because until recently, vehicle owners never knew the operating temperature and pressure of their tires. However, with the introduction of aftermarket TPMS (tire pressure monitoring systems) as used by many RV owners, they now have those numbers presented to them.
What’s missing
What’s missing is two things. First is training by the selling dealer as to what inflation is needed to support the stated load. Second is an explanation of what the words on the tire actually mean.
I am not sure if the RV salesman has ever received the training other than to tell the customer the information is in the Owner’s Manual.
Hopefully, when an RV owner reads “Max Load” they understand that they should never load the tire more than that.
The confusion comes with the inclusion of the word “Max” as it relates to tire pressure.
IMO, the wording would be much better and more logical if the tire said “Max Load yyyy pounds at xx PSI cold.”
I would leave it up to the people at DOT to try and explain why they were not consistent across all types of tires with the wording on load and inflation limits, but I have no idea who to ask. They would likely pass the question off and say, “Ask the tire manufacturer.” But the manufacturer is only following the regulations established by DOT.
I have a large number of posts in my blog that mention inflation. If you have questions I can suggest you review my posts. The questions are asked a few different ways. I provide the answer with what I believe is a consistent interpretation of the intent of the requirements.
Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.
Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.
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So the “Max PSI” stamp is simply stating that’s the correct pressure for the maximum load the tire is designed to carry?
I always thought that was giving the maximum pressure the tire was designed to run safely at. I was always in the 19% believing that’s the most pressure (cold) that should ever be put in that tire.
In reading your blog “https://www.rvtiresafety.net/2017/08/how-i-program-my-tpms.html” which was very informational and educational, I have a question concerning figuring the tire loading. In my case i have a recreational trailer that has 2 axles and 4 tires. Both tires on each side are within inches of one another. My axle weight is 4450. From the Load chart for goodyear ST205/75R14 at 65psi shows a weight of 2040 (D). Do I multiply the 2040 x 4 to get my load rating or x 2 for 2 axels. What does the load chart rating mean to me when I have 4 tires. How do I figure my tire load so I can set my TPS accurately?
There is another variable rarely mentioned…the wheel (rim) also has a pressure rating. I guess, in theory, wheels are designed to always be able to withstand the pressure of a tire it is designed to have on it, so not an issue to be concerned about.
Is that always true?
If we assume that the wheel manufacturer has done responsible design and testing to some specific inflation level and that the RV company purchases wheels that are rated for the Load & Inflation numbers on the tire then I would not expect the normal change in pressure in tires to be of any concern. You see that Engineers don’t like absolutes like “Always” because we can many times think of the end-user doing something stupid, like inflating tires with Propane. But if you follow your Owner’s Manual you should not have any problems.
I use to work at a gas station back in the day when air was free. We had a air hose for anybody who needed to top off their tires. Along with the free air, we would provide a pressure guage, if the customer asked. It became an amusement event, where we kept a tally sheet on a clipboard, for every bicyclist that would blow up their tires, and they would burst. As anybody who pays attention to such things, bicycle tires reach ” pressure” very quickly, as it is a fuction of tire volume in relation to pressure of air input over time.
Well, it became evident that many cyclists never understood this basics physics principle, and so much so, that at times the burst would be so violent, the bike would flip over from the force of the burst.
It got to the point where when we were pumping gas into the customers c (the old days) and would literally stop and stare from the pump island, wondering what level of {bleeped} was filling their tires up, without the need of a guage.
Yes, as I told Spike “end-user doing something stupid”
Great explanation Roger! It’s true that with TPMS we are watching our tires for temperature and pressure and maybe even having some anxiety has we drive and watch those numbers climb. I check my pressures with the tires cold or as you stated not yet warmed by driving or sun. I set them to the stated pressure on the vehicle sticker of my rig (which happens to match the sidewall of the tire but I know this isn’t always the case). I’ve not had any issues yet 🤞
My problem has been when leaving northern Illinois in late December for warmer locations. I start at 80 psi in 20 degree weather. And as I go south, there comes a point where the tpms resdings start to exceed 90 and I get concerned.
(Of course the reverse problm occurs in late February, when going from warm to very cold.)
So the question is, what’s the acceptable level of variability?
I just had this happen in tx put 82 psi in my class a rv going from one camp ground to another 100 miles away as driving monitor start beeping reading 97 i am thinking tires getting hot because road are getting warmer 😳
Gerald Are you sure that 82 is what you should be running based on your measured weight? An 18% increase in inflation is in the “normal” range for a fully loaded tire. You might want to review Inflation and weight on my Blog http://www.rvtiresafety.net to be sure that 82 is a good “Set” pressure.
The general advice is to check the inflation at the start of every travel day. So unless you drive from Ill to Phoenix in a single day you should not have a problem. But the thing to remember is that the heat generation in a tire due to bending is a lot less when the tire is inflated to 90. Your pressure increase of 10 psi is 12% which translates to a temperature change of 60F or an Ambient air temperature of 80F which is possible in Phoenix