By Roger Marble
With thousands of new RV owners out there, the answer to the question of “How cold does it need to be for me to check tire inflation” continues to come up. This will be new information for some and a refresher for others.
Some people want to refer to a temperature compensation chart and go through some calculations to learn the “correct” tire inflation when it is 82 F or 62 F outside. This is not what you should do.
Tire pressure is not based on any laboratory standard temperature (some claim 70 F or 68 F). It is based on the tire not being warmed from either use, i.e., being driven in the previous two hours, or from being in the sun for the previous two hours. Even partial sun can affect the reading.
Classical “temperature in the shade” is the “ambient” temperature we tire engineers are talking about. Not temperature in a theoretical laboratory.
When is the best time to check cold inflation pressure
It is correct to say, “The BEST time to check cold inflation pressure, or CIP, is the FIRST thing in the morning BEFORE the day’s temp has had a chance to increase and BEFORE the sun has had a chance to shine on the tires and BEFORE you have used the vehicle.”
With many people installing a tire pressure monitoring system (TPMS) that provides a temperature reading to the driver for the first time, some are surprised when they see that both temperature and pressure increase as they drive. Please remember it is NOT correct to bleed pressure from a “hot or warm” tire after you have started that day’s travels.
As I have covered in other posts, the tire pressure will change by about 2% for each change of 10 F. There are some pages on the internet that say 1 psi for 10 F, but those are talking about standard 36 psi passenger tires, not 80 or 100 psi RV tires.
I suggest a +10% “cushion” on the required inflation
Now, if you are driving from a campground, let’s say, on top of Pike’s Peak and stop for lunch for two hours in the shade in Flagstaff where it is 90 and check your air, you might find a change of a few psi. You could adjust your pressure before continuing to Phoenix, where it is 120 F. But I don’t bother to adjust inflation by the 1 to 3 psi variation I observe day to day. In my mind that is too much work. This is one reason why I suggest a +10% “cushion” on the required inflation as that eliminates the need to chase inflation with every change in ambient of 15 to 20 F. We tire engineers know that tire temperature and pressure will increase and we have taken that into consideration when we design and test tires.
What I run my tires at
NOTE: My personal CIP is 75/80 F/R on my Class-C MH. Both of these pressures are more than 15% above the minimum pressure needed to support the measured load on each tire – so I have a nice “cushion.”
I usually wait till I am home and am getting ready for the next trip before I adjust my inflation to my personal CIP. So I simply monitor the running inflation pressure. It goes up and down as ambient temperature, driving and sun exposure changes the inflation. My TPMS will warn me of air loss – so all is good as I motor down the highway.
I hope this info helps some of the first-time RV folks out there. If you are an “ol’ timer,” you might direct the “newbies” to this post when they ask about setting tire pressure.
Have a tire question? Sign up for Roger Marble’s new Facebook Group: RV tire news, information and discussion, hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.