GMC 1995 Jimmy Owner's Manual page 285

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.)
Treadwear
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:
A, B,
and C. They represent
the tire's ability to stop
on
wet pavement as 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 is based on braking
(straight-ahead) traction tests and does not include cornering (turning)
traction.
Temperature -A, By C
The temperature grades are A (the highest), B, and C, 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 C corresponds to a level 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.
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