Most tires today have a service description branded on the side wall after the tire size. This service description consists of the two following parts:
| • | The load index |
| • | The speed symbol |
The load index is a number (usually between 75-115) which defines the tire's load capacity at maximum inflation. Higher numbers indicate a greater load capacity.
The speed symbol is a letter (usually between P-Z) which defines the speed capability of the tire. In the past, this letter may have been part of the tire size. The following tires sometimes use this letter as part of the tire size:
| • | V-rated tires |
| • | Z-rated tires |
Higher letters indicate a greater speed capability. The tire companies provide charts which define the following information:
| • | Equivalent loads and speeds for all load indexes |
| • | Speed symbols that define the tire service description |
Important: When replacing the tires, do not install a tire of a smaller size or lesser speed rating than the original tire. The speed ratings only apply at the tire's full pressure. Under-inflated tires will not meet the speed rating capability.
Refer to the chart below for the common speed rating on tires used by General Motors:
Speed Symbol | Maximum Speed (km/h) | Maximum Speed (mph) |
|---|---|---|
S | 160 | 112 |
T | 190 | 118 |
U | 200 | 124 |
H | 210 | 130 |
V | 240 | 149 |
Z | Over 240 | Over 149 |
