According to PopularMechanics.com, tire pressure ratings are set by vehicle manufacturers and can be found in the owner's manual or on the vehicle itself. Most vehicles have a label or placard on the door jamb that tells the correct PSI.
Tire pressures are set by vehicle manufacturers, not tire companies. Tire companies can set the maximum tire pressure for the tire, but the vehicle manufacturer sets the proper specifications for the particular vehicle. The tire pressure should be listed in the owner's manual or on the vehicle itself, typically on a label in one of the front doors of the vehicle. An owner should open the vehicle door to find the label. If this information is not available, owners can contact the manufacturer for more information.