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Friday, August 20, 2021

How cold does it have to be to set tire pressure correctly?

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. So 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 82F or 62F outside. This is not what you should do.

Tire pressure is not based on any laboratory standard temperature (some claim 70 F or 68F) but is based on the tire not being warmed from either use, i.e. being driven in previous two hours, or from being in the Sun for previous two hours. Even partial sun can affect the reading.

Classical "Temperature in the Shade" is the "Ambient" we tire engineers are talking about. Not temperature in a theoretical laboratory.

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 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 days travels.

As I have covered in other posts, the tire pressure will change by about 2% for each change of 10F. There are some pages on the internet that say 1psi for 10F but those are talking about standard 36psi passenger tires not 80 or 100 psi RV tires.

Now if you are driving from the campground 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 20F. We tire engineers know that tire temperature and pressure will increase and we have taken that into consideration when we design and test tires.

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 which 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.



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