A letter identifies the load index for most light trucks. These letters being B. C. or D. A tire with a B load rate is restricted to a load specified at 32 psi. Where a greater load-carrying ability is required, load rate C or D tires are used.

Passenger vehicle tires come with a service description added to the end of the tire size. These service descriptions contain a number, which is the load index, and a letter, which indicates the speed rating. The load index (fig. 8-32) represents the maximum load each tire is designed to support. Because the masimum tire load capacity is branded on the sidewall of the tire, the load rate is used as a quick reference. Speed ratings (fig. 8-33) signify the safe top speed of a tire under PERFECT conditions.

Maximum Inflation Pressure
The maximum inflation pressure. printed on the sidewall of a tire, is the highest air pressure that should be induced into the tire. The tire pressure is a "cold" pressure and should be checked in the morning before operating the vehicle.

In most parts of the world, fall and early winter months are the most critical times to check inflation pressures because the days are getting colder. And since air is a gas, it contracts when cooled. For every 10 F change in ambient temperature, the inflation pressure of a tire will change by 1 psi. It will go down with lower temperatures and up with higher temperatures. The typical difference between summer and winter temperatures is about 50 F that results in a loss of 5 psi and will sacrifice handling, traction, durability, and safety.

Tire Grades
The Department of Transportation requires each manufacturer to grade its tires under the Uniform Tire Quality Grade (UTQG) labeling system and establishes ratings for treadwear, traction. and temperature resistance (fig. 8-34). These tests are conducted independently by each manufacturer following government guidelines to assign values that represent a comparison between the tested tire and a control tire. While traction and temperature resistance ratings are specific performance levels, the treadwear ratings are assigned by the manufacturers following field-testing and are most accurate when comparing tires of the same brand. Tire grades are as follows:

Figure 8-32.- Load index chart for a passenger vehicle.

Figure 8-33.- Speed rating chart for a passenger vehicle.

TREADWEAR- Treadwear receives a com-parative rating based on wear rate of the tire in field-testing following a government-specified course. Treadwear is given as a number: 100, 120, or 130, for instance. The higher the number, the more resistant the tire is to wear. For example, a tire grade of 150 wears 1.5 times longer than a tire graded 100. Actual performance of the tire will vary significantly depending on conditions, driving habits, care, road characteristics, and climate.

TRACTION- Straight-a-head wet braking traction has been represented by a grade of A, B, or C with A being the highest. In 1997 a new top rating of "AA" was introduced to indicate even greater wet braking traction. Traction grades do NOT indicate wet cornering ability.

TEMPERATURE- Temperature resistance is grades A, B. or C. This represents the resistance of the tire to heat generated by running at high speed. Grade C is the minimum level of performance for all passenger vehicle tires as set under Federal Motor Vehicle Safety Standards. This grade is established for a tire that is properly inflated and not overloaded.

NOTE
Uniform Tire Quality Grade ratings are NOT required on winter, light truck, and commercial tires.