Tires carry a wealth of information on their sidewalls; unfortunately, it's all in code.

Tires carry a wealth of information on their sidewalls; unfortunately, it's all in code. Most of it is of no concern to many owners, but it can be valuable for those who want to know the seasons for which their tires are intended, those who plan to replace their tires, or new-car buyers who are given a choice of sizes and types.

Apart from the name of the manufacturer and the tire model -- such as Bridgestone Turanza EL 400 -- the following marking, from a Honda Civic, is likely the most prominent.

P simply means this is a passenger-car tire. Other codes include T (temporary, as in a spare tire), LT (light truck) and C (commercial). Not all tires include this letter.

205 (the first number) is the tire's width in millimeters.

55 (the number following the slash) is a measure of the tire's profile -- the height of its sidewall relative to its width -- expressed as the sidewall's aspect ratio in a percentage. In this example, the sidewall height is 55 percent of the tire's width, which is 205 mm. The number is known as the tire's series; a sporty tire with shorter sidewalls would be considered a lower-series tire. Off-road tires tend to be higher-series, designed to absorb the impact from rough surfaces.

R means it is a radial tire, which describes the way the underlying layers, or plies, are laid out. The radial design is now so pervasive that the R designation is superfluous -- though it conveniently separates the series and diameter numbers. On some performance tires, an additional letter precedes the R: A ZR designation means the tire's maximum speed rating is above 149 mph (see the key below). This is another unnecessary character; all tires provide a letter designation in the service description, explained below. It follows the size code, sometimes in smaller type.

Speed Ratings Letter / Max. MP