Air Pressure

Specifically the air pressure of the tires. Changing the air pressure in the tires is a way to alter the setup and handling of a NASCAR race car. Different tires are set to different air pressures to adjust for the conditions of the track. For example, if the race car is tight coming off a corner, a driver might request an increase in air pressure in the right rear tire to "loosen it up."

Force exerted by air within a tire, expressed in pounds per inch (psi).

The cumulative force exerted on any surface by the molecules composing air.