Originally Posted by Adam_HybridCivic

40 psi means 40 units of pressure per square inch(not that you didn't know that), therefore the air inside the tire is exerting 40 psi against the walls of the tire. When the car is then put under load or sat to the ground the amount of pressure in the tire does not change. However, the shape of the tire that holds the air changes shape still exerting 40 psi. When the tire is under load however, it must have more air in the tire to keep the same `shape when not under load.