1998 Chevrolet Astro Owner's Manual - Page 309

Uniform Tire Quality Grading The following information relates to the system developed by the United States National Highway Traffic Safety Administration, which grades tires by treadwear, traction and temperature performance. (This applies only to vehicles sold in the United States.) The grades are molded on the sidewalls of most passenger car tires. The Uniform Tire Quality Grading system does not apply to deep tread, winter-type snow tires, space-saver or temporary use spare tires, tires with nominal rim diameters of 10 to 12 inches (25 to 30 cm), or to some limited-production tires. While the tires available on General Motors passenger cars and light trucks may vary with respect these to grades, they must also conform to Federal safety requirements and additional General Motors Tire Performance Criteria (TPC) standards. Treadwe: The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test course. For example, a tire graded 150 would wear one and a half (1 1/2) times as well on the government course as a tire graded 100. The relative performance of tires depends upon the actual conditions of their use, however, and may depart significantly from the norm due to variations in driving habits, service practices and differences in road characteristics and climate. Traction -- A, B, C The traction grades, from highest to lowest, are B, A, and C, and they represent the tire's ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces asphalt and concrete. of A tire marked C may have poor traction performance. Warning: The traction grade assigned to this tire is based on braking (straight ahead) traction tests and does not include cornering (turning) traction. - 6-47