As you drive, the tire pressure of each tire rises in proportion to the rise of air temperature inside the tire. The temperature of that air is affected by 4 sources:
A. The rubber of a rotating tire generates its own heat from the friction of the rubber flexing.
B. The rubber and wheel gain heat from any direct exposure to the sun.
C. Heat transfers from asphalt to rubber, or from rubber to asphalt, depending on which one is warmer, in proportion to the difference in temperature. This is a big factor when driving in an area like the Mojave Desert during a summer day where the asphalt can exceed 160°F (71°C).
D. Heat transfers to or from the air surrounding the wheel, depending on which one is warmer.
(B) and (C) are more pronounced in places where the sun is more intense. But even without the sun, it is fairly typical that a tire's internal air temperature (and therefore its pressure) rises significantly from the heat gained during driving. This is normal and the tire manufacturer takes these things into account in the tire's design.
