1998 Chevrolet Lumina Owner's Manual - Page 282

Treadwear The treadwear gradeis a comparative rating based on the wear rateof the tire when tested under controlled conditions on a specified government test course. For example, a tire graded would wear one and a 150 half (1 1/2) times as well on the government course as a tire graded 100. The relative performance tires of 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 pavementas measured under controlled conditions on 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 based is on braking (straight ahead) traction tests and does not include cornering (turning) traction. -- Temperature A, B, C The temperature grades are (the highest), B, and C, A representing the tire's resistance to the generation of heat and its ability to dissipate heat when tested under controlled conditions on a specified indoor laboratory test wheel. Sustained high temperature can cause the material of the tire to degenerate and reduce tire life, and excessive temperature can lead to sudden tire failure. The grade corresponds to a level C of performance which all passenger car tires must meet under the Federal Motor Vehicle Safety Standard No. 109. Grades B and A represent higher levels of performance on the laboratory test wheel than the minimum required by law. Warning: The temperature gradefor this tire is established for a tire that is properly inflated and not overloaded. Excessive speed, underinflation, or excessive loading, either separately in combination, or can cause heat buildup and possible tire failure. --