When vehicle manufacturers select a tire size for a vehicle, they evaluate the vehicle's gross axle weights, the anticipated use of the tire, and the tire diameter and width. Adjustments to these factors give the manufacturer a way to improve handling and appearance. This is especially true for performance tire sizes. The size selected is rarely limited to only one capability (i.e. carrying the vehicle's weight).

The tire usually needs to have additional load capacity as well. This extra capacity is important because without it all of the tire's performance would be used up just carrying the weight of the vehicle and little would be left for durability at high speeds or responsive handling. For all vehicles produced since 1968, the original tires sizes and inflation pressures (including the spare) are listed on a vehicle placard. This placard can be located on:

* The driver-side door or door jamb (Ford vehicles on the rear passenger door jamb)
* Glove box or counsel door
* Fuel filler door
* The engine compartment



Additionally, some manufacturers also list the original tire pressure in the vehicle's owner's manual. If a car's inflation pressure has varied from that which was recommended by the manufacturer, it's likely that the tire's wear and performance characteristics have also changed.