Ford 1998 Windstar Owner's Manual page 297

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 one-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,
and 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 B 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 Stadard 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 grade for this tire is established for a tire that is
properly inflated and not overloaded. Excessive speed,
underinflation, or excessive loading, either separately or in
combination, can cause heat buildup and possible tire failure.
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