By Roger Marble
A few times each week there are posts on various RV forums asking about tire inflation. There continue to be many readers confused by the words “Max Cold Inflation” on the sidewall of many tires. This wording, while confusing, is mandated by regulation from DOT so don’t blame the tire companies. You are more than welcome to write to the National Highway Traffic Safety Administration, 1200 New Jersey Avenue SE, Washington, D.C. 20590. Ask them why they require the marking saying “Max xx psi” when they know that all tires warm up when in use. Also, that many will have inflation above the stated “Max” soon after we start driving. Furthermore, many people then bleed the tire pressure down to get below the stated “Max”. This bleeding of hot air pressure has resulted in numerous tire failures.
As readers of my blog, you know you should NEVER bleed down the pressure in hot or warm tires. The only time you might lower tire pressure is after you have been parked away from direct sunlight and for at least two hours, and have moved to a location that is much warmer than where you were parked the day before your travels.
Don’t adjust tire pressure for future conditions
Despite the information in my posts, some folks want to play the game of trying to adjust the tire pressure for the location they are traveling to. This is not the proper method of establishing your goal “set pressure” even if the expected “future” ambient is significantly different than the ambient where you are starting your travels.
I’m not sure why this topic seems difficult to understand but maybe I’m just too close to the topic to see the confusion. I have more than 25 blog posts that mention “Cold Inflation”. This link will display a dozen of those posts if you need a review.
I suppose one problem may be that with the introduction of the TPMS, many people are now getting a bit of “information overload” as they watch the pressure and temperature readings from each tire go up and down as they drive. This may be the TMI (too much information) I spoke of in the title of this post.
Set tire pressure the morning of your travel day
I suggest you just stick to setting the pressure on the morning (before driving and generally before sunlight has hit the tires) of your travel day. Don’t try and second-guess what the ambient temperature will be tomorrow and hundreds of miles away.
I have posts on how I suggest you learn what your “Set Pressure” should be. The procedure is a little different for motor vehicles (cars, trucks and motorhomes) vs. trailers.
Generally, trailers need to be running higher inflation and/or lower load levels than motor vehicles because of the higher level of Interply Shear that is inherent in multi-axle trailers.
For all users, I advise that the “set” pressure be at least the minimum in the load inflation tables for the measured load +10%.
For multi-axle trailers, I suggest you follow the inflation shown on the Certification Sticker. But you should confirm your actual axle load is no more than 85% of GAWR if you want a chance of getting better tire life.
Motorhomes should first follow inflation on Certification Sticker
Motorhomes should follow the Certification Sticker inflation until they have confirmed the actual loads on their tires. Then, consulting the Load Inflation tables, identify the MINIMUM tire pressure they should ever run. I recommend you then add at least 10% to that inflation with a +15% Reserve Load capacity being better.
Remember, we are trying to always protect and prevent the tires from ever being in an overload or low inflation pressure condition when we are setting the temperature.
A tire with inflation higher than x psi will generate less heat than the same tire with the inflation lower than x. As you drive and your tire heats up, the pressure will rise. But with the increase in pressure, the amount of heat generated will decrease so the pressure will stabilize.
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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Roger, I enjoy your articles, but find some reader comments to be amusing. As I grew up many times I was told “don’t over-think it”. Read the sticker, air the tires. Some seem to think that they are racing at Daytona, I guess. You are right, there never were these discussions before TPMS. Too much info bombards us every day – but thanks for keepin’ us rollin’.
The best bet for 99% of people is just air your tires to what the little yellow sticker says on their door jam, trailer door or wherever it’s located each morning before travel. Overall to get to the absolute required pressure is confusing and most do not weigh their vehicles correctly anyway. I know my overall axle weights for truck and 5er but have never weighed each wheel separately. I try to load each side evenly.
Since I went from “D” rated tires to “E” rated tires on my trailer, the tire pressures listed on the Certification Sticker are off. I now just set the tires at 80 psi (the max load mentioned on the tires) and go from there. Staying under 60 mph and this routine has served well for over 25 years. I don’t have a TPMS on the trailer but DO on my truck. I cringe all day as I watch the tire pressure go up as we chug along.
Thank you Roger. I appreciate your frequent reminders but rather than discuss blaming the NRTSA and letter writing campaigns just say out loud:
The MAX number on the tire is NOT how you determine the correct pressure of your tires!
It is a do not exceed number (max? Doh!)
I do like the reminder that it is the MAX cold pressure.
I also believe that too many people (and RV builders) cut it too close. If you calculate the actual PSI for your tires and it is over or close to the MAX then you have the wrong tires!
Pay attention to the speed rating and the weight rating codes for every tire, before you buy them.
Tim, I may be reading your comment incorrectly but when you say “It is a do not exceed number” it sounds like you may have missed the point of the post. The “Max Pressure number on the tire sidewall is only stating that increasing the inflation above that number will NOT further increase the load capacity of the tire. The tire can easily exceed that pressure when driving down the road. I do agree with you that too many “cut it too close” on load capacity.
Pulled into Arizona rest area. Guy was messing with his pick-up tire. I asked if he needed air, I carry a 12v air compressor that works well for me. He said, “No, he was letting air out to adjust for the change in altitude.” Strange to me concept.
Tom, I am sorry to report that many people seem to incorrectly think that tire failures are the result of too high a pressure in their tires. This is just not true. Undamaged tires can tolerate a significant increase in pressure. If people knew from scale reading that the actual load on their tires was lower than the stated “Max Load” number on the tires…AND they knew from scale readings they were not exceeding the GAWR….. AND set the tire pressure when the tire was at AMBIENT temperature, there is no reason to be messing with tire pressure as long as their TPMS is not indicating a leak.