Ok, I tried to be quiet, but my chemical engineering background won't let me.
Air in your tires (or nitrogen, argon, hydrogen, hellium, whatever) obeys the
ideal gas law, PV=nRT. The tire volume is fixed, so the absolute pressure
increases in direct proportion to the absolute temperature. Let's say you
have 20 psi in your tires at 70 degrees F. The absolute temperature is 70 +
460 = 530 degrees R and at sea level the absolute pressure is 20 + 14.7 =
34.7. If the temperature drops to 50 deg F (510 deg R), the pressure will
drop to 34.7(510/530) - 14.7 = 18.7 psi. If the temperature increases to 130
deg F (590 deg R) during a race, the pressure goes to 34.7(590/530) - 14.7 =
23.9 psi.
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