The numbers and letters engraved on a tire reflect, among other things, its load bearing and speed handling capacity. These are important specifications provided by the manufacturer, and are a part of that tire’s rating.
All tires have alpha-numeric characters engraved on their inner sides. These engravings are specific codes, which provide the user with information on the dimensions of the tire and its key limitations, such as load-bearing ability, the maximum speed it can achieve, temperature and traction.
Starting from 1991, all manufacturers have been marking the service descriptions on the tires. The following section explains what speed rating and load index imply, and also provides tables representing the typical values of both these ratings found on most tires.
Tire Rating Charts: Speed Rating and Load Index
The load index is an assigned numerical value, that identifies the tire’s ability to carry load. The higher the tire’s load index, the greater will be its capacity to carry load. The load indexes of the tires used on passenger cars and light trucks, typically range from 70 to 110.
|Load Index||Pounds||Load Index||Pounds|
The speed rating is indicative of the tire’s capacity to move at high speeds. Most of the ratings are based on laboratory tests, where the tire is pressed and rotated against a large diameter metal drum to simulate load and determine its speed. Do note however, that no matter how high the speed rating of a tire is, one shouldn’t exceed the legal speed limits.
The code for speed rating, is made up of one or two letters, or one letter and one number, and indicates the maximum permitted speed that the tire can sustain, without being damaged.
|E||43||70||Z||over 149||over 240|
|G||56||90||(W)||over 168||over 270|
|K||68||110||(Y)||over 186||over 300|
Most tires use the ISO Metric sizing system for their specifications. The European Tyre and Rim Technical Organization (ETRTO) and the U.S. Tire and Rim Association (T&RA) are the two major standardizing organizations for rating speed and load indexes of tires.