Most tires today have a service description branded on the side wall after the tire size. This service description consists of two parts: The load index and the speed symbol. The load index is a number, usually between 75 and 115, which defines the tire's load capacity at maximum inflation. Higher numbers mean greater load capacity. The speed symbol is a letter, usually between P and Z, which defines the speed capability of the tire. In the past, this letter may have been part of the tire size. This continues to be the case with some V and Z rated tires. The higher the letter, the greater the speed capability. The tire companies have charts which define equivalent loads and speeds for all load indexes and speed symbols that make up the tire service description.

Important: When replacing tires, never install a tire that is a smaller size or lesser speed rating than the tire that is used as original equipment. These speeds only apply at the tire's full pressure. An under-inflated tire will not meet its speed rating capability.

The following chart shows the common speed rating on tires used by General Motors.