Pontiac 1993 Grand Am Owner's Manual page 237

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, 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 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 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.
Those grades are molded on the
sidewalls of passenger car tires.
While the tires available as standard or
optional equipment on General Motors
vehicles may vary with respect to these
grades, all such tires meet General
Motors performance standards and have
been approved for use on General
Motors vehicles. All passenger type
(P Metric) tires must conform to Federal
safety requirements in addition to these
grades.