Chevrolet 1993 Astro Passenger Owner's Manual page 285

If you ever replace your tires with those not having a TPC Spec number,
make sure they are the same size, load range, speed rating and construction
type (bias, bias-belted or radial) as your original tires.
A
Mixing tires could cause you to lose control while driving. If you mix
tires of different sizes or types (like radial and bias-belted tires) the
vehicle may not handle properly, and you could have a crash. Be
sure to use the same size and type tires on all wheels.
I
It's all right to drive with your compact spare, though. It was developed for
limited use on your vehicle.
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 (142)
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.
Temperatur+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
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