Most tires have a service description branded onto the sidewall following the tire size. This service description consists of two parts:

The load index is a number normally 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, the speed symbol may have been part of the tire size. This practice continues on some V rated and Z rated tires. The higher the letter, the greater the speed capability.

Tire companies have charts which define equivalent loads and speed for all load indexes and speed symbols that make up the tire service description.