And, the tires heat up the more you drive, which will also affect the air pressure reading.
I always check tire pressure on a cold tire.
Air (gas) expands as it is heated and contracts as it is cooled. Thus driving, sunny days, etc...will expand the air pressure in the tire. Just like cold weather can cause tires to go flat (ask about 4 of my female coworkers that ignored their tire inflation light...as temps began to drop...and they all ended up with flat or nearly flat tires in the parking lot).
