Explain Tire Sizes
Tires come in many different sizes and types, depending on the size and type of the vehicle. The type and size of the tire is explained by a set of numbers and letters printed on the side. This series starts with a single letter, a pair of numbers separated by a slash, another letter, another pair of numbers separated by a space and one final letter.
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Types
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The first letter generally describes the type of vehicle for which the tire is designed. Tires designed for passenger vehicles will have a "P" in this area. Others include "C" for commercial, "LT" for light trucks and "ST" for special trailers. A "T" is used for temporary spare tires.
Width and Height
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The first number in the series should have three digits; this indicates the tire's widest point, measured in millimeters. The second number, following the slash, will have two digits and indicate the tire's height from the rim to the edge. This is actually measured by the percentage of the width. A tire labeled 225/50 measures 225 millimeters wide and 112.5 millimeters high--50 percent of the width.
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Construction
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The second letter indicates how the tire is constructed. An "R" in this spot stands for radial construction, and this is the most common tire construction. Other examples include a "D" for a bias ply construction and a "B" for tires that are belted.
Wheel Size
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The last number before the space indicates the size of the wheel for which the tire is designed. This measurement is recorded in inches. A 14 in this place means the tire is meant for a 14-inch wheel. Most wheels range from 13 to 18 inches, although aftermarket wheels of at least 22 inches are possible.
Load Capacity
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The number that follows the space indicates the tire's load capacity. This number is a rating for the maximum load the tire can carry. This number requires a tire guide's load index chart to completely decipher, but passenger tires usually range from 71 for 761 pounds to 110 for 2,337 pounds.
Speed Rating
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The final letter is the speed rating of the tire. This works with a tire guide's speed rating chart to determine the maximum speed a car should be driven with this tire. "M" is the lowest rating for 81 mph, while "Y" is the lowest rating for 186 mph. If the last number and letter include a Y and are in parentheses, the tire has been tested at more than 186 mph.
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