[obermd]

Yup. Air in the tires expands and contracts with temperature. Best thing to do is check when cold, 3-4 psi change with temperatures is normal. Only way around the psi difference is to air up with nitrogen and get all atmospheric air out. But that technique is rife with opinions....

Unless you're driving a race car or a jet aircraft, don't bother with nitrogen. Nitrogen expands (increases pressure in an enclosed container) and contracts (decreases pressure in an enclosed container) with temperature changes at the same rate as all other gasses. Where nitrogen has a very very miniscule edge over the normal atmospheric gas mix is in corrosion. When I say miniscule I mean you might see corrosion inside the wheels after a decade using humid, salt laden air found in the tropics near an ocean. In this situation, however, you'll probably have other corrosion and rust problems long before the wheels start to corrode.