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Originally Posted by PopBeavers
On my last trip I measured the tire pressure twice before starting.
1. In the garage when the outside temperature is 95 degrees at noon. TM was in the garage overnight. My guess is the temperature was 75. Pressure was 50.
2. Pulled the TM out of the garage, drove around the block once and parked it for an hour. Perssure went up 8 degrees so it was now at 58.
Since out in the sun seemed to more reasonably represent ambient temperature than parked in the garage overnight I let some air out.
Now the really interesting question. At 5pm the outside temperature was 108 degrees. I can't really measure the tire pressure because I just drove for 4 hours and the tires were obviously hot. Should I have let some air out of the tires? because the ambient temperature had risen 13 degrees?
Suppose you start out with properly inflated tires with the outside temperature around 50 in the mountains early in the morning and then drive to Death Valley in July where the temperature is 125. What should you do? It seems to me that you should let some air out of the tires but how much? When travelling, how long should you park to let the tires cool before measuring the pressure? There must be a rule of thumb to deal with drastic temperature changes. My simple test shows that a temperature increase of 20 degrees (garage to street) increases the tire pressure by 8 pounds (assuming the trip around the block with an hour to cool down had no effect on pressure).
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.........one PSI per 10 deg F increase of Outside Air Temp (OAT). Therefore, if it had been pressurized to 50 psi at 75 deg F, it should rise to 52 psi at 95 deg F. However, a black tire exposed to the sun can accumulate a lot of additional heat due to thermal inSOLation.
The 'round the block' driving shouldn't have made too much difference, but let's say it raised the temp 1 deg F. Then the tire pressure would have been 53 psi. An hour later, outside, in the sun the pressure is 58 PSI. Assuming the original Cold air temp was 75 deg F, then the sun heating the tire added another 50 deg. F to the air temp in the tire.
What to do?: When the tire is cold, set the pressure to 50 psi. If 50 deg F, set it to 50 psi. If 100 deg F, set it to 50 psi. If you know what the cold temp was when last adjustment to pressure made, then when traveling into death Valley (per your example), remove 1 psi for each 10 deg F greater temp. I.e.; (125-50)/10 = 7.5 psi reduction. Though the tires will be hot from travel, the cold temp is now 125 deg F. [Of course, by the time the tires dissipate the heat generated whilst towing, the local OAT will have dropped. Even Death Valley cools at night]
Denny_A