Air preasure

[boatman]

Leaving on first trip of the year to Nascar race in Bristol TN Thursday my question is I set the air preasure in my tires Saturday when it was 45 degrees at 80 psi today it's 80 degrees so being courteous I checked them again they were at 85 psi if they gain that much in 40 degrees what does cold temp mean .

[ependydad]

Cold temp means after the tire has been sitting and before you get going. If the day's travels start at 45, cold temp is 45. If the day's travels are 85, cold temp is 85.

[DaGrinch]

This is known as Charles's Law. As a gas warms, it expands - the volume increases, thus creating more pressure. In reverse, as it cools, it looses volume and there is less pressure.
The mere fact they expanded 5psi in a 40 degree temp change just sitting would indicate to me to check the pressures just before I left, so as to not have the pressure expand too much once on the road. Being a race fan, you know tire temps can adversely affect tires and tire wear.