On the sidewall of your car's tire is a series of letters and numbers that typically look like this: P185/60R16 89V. These seemingly random numbers are not random at all. They actually tell you everything you need to know about your tire so you know exactly what to look for when buying a new or replacement tire for your car. The information that is included in this series of letters and numbers is the tire's intended purpose, its dimensions, its load capacity, and its high speed durability.

Service Type

Most tire sizes start out with a letter, as mentioned above. These letters can be: P, T, LT, or ST. The letters indicate the service type of the tire, or the intended use of the tire. A "P" means the tire is designed for passenger vehicles such as cars, minivans, and SUVs. These are the most frequently used type of tires.

A "T" means that the tire is a temporary spare tire which was designed only to be used temporarily, until a flat tire can be fixed or replaced.

An "LT" means the tire is a light truck tire and is intended to be used on automobiles that are capable of carrying heavy cargo or towing large trailers, such as pick-up trucks, SUVs, and full-size vans.

Sometimes "LT" will be at the end of the tire size. When this is the case, the tire is supposed to be used on vehicles that are capable of carrying heavy cargo and towing trailers, that use 16.5 inch diameter rims, or are oversized tires designed to help vehicles drive on top of loose dirt or sand.

An "ST" means that the tire is a special trailer service size tire that is specifically designed to be used only on boats or utility trailers.

Sometimes there is no letter at all at the beginning of the tire size. This means that the tire is a Euro-metric size tire. Euro-metric tires originated in Europe and are primarily used on European cars. They are dimensionally equivalent to P-metric tires, but sometimes differ in load carrying capabilities. As a result, Euro-metric and P-metric tires are interchangeable if the axle has two tires of the same metric, and if the load index and speed rating are the same (more on load index and speed ratings below).

If a Euro-metric tire ends with a "C", then the tire is a commercial tire that is for use on vans or delivery trucks capable of carrying heavy loads.

Section Width

The 3 digit number following the Service Type indicates the tire's section width, or the measurement from one sidewall to the other sidewall, in millimeters. So a section width of 225 means that from sidewall to sidewall is 225 millimeters.

Sidewall Aspect Ration

After the Section Width is a 2 digit number. This number represents the tire's Sidewall Aspect Ration, also called the Sidewall Height, Profile, Or Series, from the rim to the tread, as a percentage of the Section Width. So a 50 indicates that the tire's sidewall height is 50% of its section width.

Internal Contruction

Following the Sidewall Aspect Ration is another letter. This letter indicates the tire's internal construction, which can signified by an "R", a "D", or a "B". An "R" means that the tire has a radial construction, meaning the tire's body plies radiate out from the imaginary center of the tire.

A "D" means the tire has a bias ply construction. This means that the tire's plies crisscross on a diagonal.

A "B" means that the tire is belted. This means that the tire's plies are not only crisscrossed on a diagonal, but they are also reinforced with belts under the tread.

Tire and Wheel Diameter

The 2 digit number after the Internal Construction signifies the tire and wheel diameter in inches. So a 16 means that the diameter of the tire is 16 inches.

Load Index and Speed Ratingt

The 2 digit number and letter following the Tire and Wheel Diameter represent the tire's load index and speed rating. The number corresponds to the load carrying capacity of the tire. The higher the number, the more weight the tire can carry. You can view a chart of what number corresponds to what weight by searching for "Load Index Chart" online.

The letter represents the speed rating of the tire, or the maximum speed that the tire can handle. The closer to the end of the alphabet the letter is, the higher speeds the tire can handle. Just as with the Load Index Chart, you can also view a "Speed Rating Chart" online.