1997 GMC Sonoma Owner's Manual - Page 287

Uniform Tire Quality Grading The following information relates to system the 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 to these grades, they must also conform to Federal safety requirements and additional General Motors Tire Performance Criteria (TPC) standards. Treadwear The treadwear gradeis a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specifiedgovernment test course. For example, a tiregraded 150 would wear one and a half ( 1 1/2) times as well on the government course as a tire graded 100. The relativeperformance 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 A,B, and C, and they represent the tire's abilityto stop on wet on pavement as measured under controlled conditions specified government test surfaces of asphalt and concrete. 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. 6-49