Need help understanding your tire size?  Your tire size is an alphanumeric code found on your tire sidewall.  An example size is P185/60R14 82H.  Each number and letter represents a specific measurement:


Tire Sidewall Diagram


P Designates the type of vehicle the tire is designed to fit: passenger (P), light truck (LT), temporary spare (T), and special trailers (ST). No letter indicates that the tire is euro-metric.
185 Identifies the tire section width, which is the measurement of the tire from sidewall to sidewall in millimeters.  This measurement varies depending on the rim to which the tire is fitted. (There are 25.4 millimeters per 1 inch.)
60 Is the two-figure aspect ratio.  This percentage compares the tire's section height with the tire's section width.  For example, the aspect ratio of 60 meaNs that the tire's section height is 60% of the tire's section width.
R Is the letter that stands for the type of internal construction of the tire. R stands for radial construction. Almost all tires have radial construction. B means belted bias, and D stands for diagonal bias construction.
14 Is the last dimension listed in the size and represents the diameter of the wheel rim, which is most often measured in inches.
82 Refers to the load index and speed rating, or service description. It's the number that follows the tire size. The load index tells you how much weight the tire can support when properly inflated.  Load indices range from 74 to 150 for passenger tires with each numeric value correspondidng to a certain carrying capacity.  The carrying capacity for each value can be found on a load index chart.  On each U.S. passenger car tire, the load limit is listed in pounds. 
H Designates the speed rating, represented by letters ranging from A to Z. Each letter coincides with the maximum speed a tire can sustain under its recommended load capacity.  For example, S is equivalent to a max speed of 112 mph. Even though a tire can perform at this speed, we do not advocate exceeding legal speed limits.