The original tire size is available in a number of places including in the owner's manual, on the glove box lid, inside the fuel hatch and in the driver's side doorjamb. A vehicle's original tire size is also available by contacting the dealership or a tire retail store.
The first three numbers of a tire size indicate the width of the tire in millimeters. The next number, usually a two-digit number, is the aspect ratio, which indicates the relationship between the height and width of the tire. A letter R next to the aspect ratio means that the tire has radial construction, which helps to provide stability, strength, ride comfort and flexibility. The two-digit number following the R indicates the wheel diameter.
Following the wheel diameter is a space and then another number, which is the load index. Ranging from zero to 279, the load index indicates how much weight the tire can carry at maximum inflation. The letter that follows the load index designates the tire quality, ranging from a low rating of Q to a high rating of Z. Finally, if a letter M or S is present, then the tire meets certain standards for mud or snow. All-season tires have both letters.