Calling all tire experts

[Jon ghiles]

OK, all you tire experts out here in Newell Land here's a tech question, from a non-tech (challenged really):

I understand cold air is denser than hot air. Does that mean that on a cool morning, one's tire pressure would measure less over another?

For example, if I check my pressure early in the morning and its say 100 psi when normally I run 120. Do I bring the pressure up to 120 or leave it alone?

[folivier]

Most tire manufacturers say to adjust your pressure, BUT, the air didn't go anywhere. It's still there, and when the temp. warms up you will be overpressured.
Another thing is when the sun is shining on one side the pressures on those tires will be higher than the ones in the shade! What to do?
I usually air up my tires when they are all shaded and the temp is around 70. If the temp is 50 in the morning my tires will be 5-10 psi lower. But if I bring them up then later in the day they will be too high. So I don't worry about the difference.
And yes I do understand Boyles law.

[NewellCrazy]

Internal tire pressures change about 2% for every 10 deg. At the tire pressures we work with that's pretty close to 2 psi for every 10 deg F.

Technically you should pump you 100 psi tire back up to 120. In reality, temp swings aren't usually this drastic, so one can usually find a pressure that works under normal temperature swings.

Hope this helps,

Sean