The first letter on the tire is the sizing system. For American cars and light trucks, this is usually "P" for passenger metric. You might see a "T," which stands for temporary, on the spare tire in your trunk.
Section Width and Aspect Ratio
The first three numbers are width of the tire at the widest point in centimeters. The next two numbers (after the first slash) represent the aspect ratio. This is the height of the tire as a percentage of its width.
Material and Diameter of rims
The next letter (optional) refers to the outer construction of the tire. For example, "R" means radial. The next two numbers represent the diameter of the rims in inches.
Load Index and Speed Rating
The last two numbers are the load index, and the last letter is the speed rating. These tell you how heavy a load the tires can carry and how fast the tires are able to travel. You must cross-reference the load index and speed rating with a table available from your local tire shop, or from online tire resellers (see link below to the Tire Rack).
The numbers on the sides of American tires are standardized by the Tire and Rim Association of America, which was established in 1903 under a different name and offers a variety of publications.
The numbers printed on tires give information about the size, type and capacity of the tire. There are seven descriptors: Sizing system, width, aspect ratio, tire material, diameter of applicable rims, load index and speed rating. The format is usually X### / ##X## / X##, where an "X" is a letter and "#" is a number.