Usually, tire names begin with a letter which signifies the type of vehicle it is designed for. Some common letters include:
This three-digit number is the section width, or cross section, in millimeters.
This two-digit number indicates the height of the sidewall from rim to tread, expressed as a percentage of the tread width. A lower aspect ratio indicates a shorter sidewall, and a higher aspect ratio indicates a taller sidewall.
This letter indicates the internal construction of the tire. Some common letters include:
The number following the internal construction letter signifies the tire and wheel diameter and it is measured in inches. Some common measurements are 8, 10, 12, 13, 14, 15, 16, 17, 18, 19, 20, 22, 23, 24, 26, and 28. These are used mostly on sedans, minivans, and vans. Sometimes tires will have a rim diameter measured in half inches (14.5, 15.5, 16.5, 17.5, and 19.5). While not as common, they are sometimes used on heavy duty vehicles such as trailers, trucks and box vans. Rim diameters that are expressed in millimeters are called millimetric sizes (these are three digits). It is very important that the tire and wheel diameters match before the tire is mounted on the wheel.
Off to the side of the tire name is the service description, which indicates the load index and speed rating.
Load index indicates the relative load carrying capabilities of a tire (how much weight it can support). Passenger cars and light trucks will usually have tires with a load index anywhere between 70 and 110. For example, a load index of 77 indicates a load carrying capacity of 908 pounds. Multiply that by the number of tires on the car, and you have 3,632 pounds (if you have four tires).
Usually indicated by a letter, speed ratings indicate the max speed recommended by the manufacturer and are established to match the speed capability of the vehicles to which they are applied. However, despite what the speed rating says, it is not recommended to go over the legal speed limit in your designated area. Some common letters used: