Tire sizes include a wealth of information on the sidewall: a series of letters and numbers indicate the class of vehicle and maximum weight tires can hold, along with a speed rating, tire dimensions and more. Tire size numbers have different letters and numbers corresponding with their individual designs. However, they all include the same set of information to help drivers find the best fit for their cars.
On American tires, drivers see a letter that begins their unique tire codes. These letters represent the appropriate vehicle class, which is "P" for passenger cars and "T" for truck. European tire brands, in contrast, do not have a letter designating class.
Next, drivers find two numbers: a tread width, measured in millimeters, and an aspect ratio, which measures the height and width of the sidewall.
Two letters follow these measurements, which indicate speed rating and radial design. Speed rating corresponds with letters. A "Z" indicates tires can withstand high speeds, while an "S" shows vehicles should not exceed 112 miles per hour. An "R" following the speed rating stands for "radial," which indicates tires have a radial construction.
Wheel diameter, measured in inches, comes next. A load index follows, and indicates the maximum weight tires can hold. Tires with a "Z" speed rating have another letter at the end, which indicates in miles per hour the top speed at which drivers can safely go.