Ford 1996 Windstar Manual page 332

Tires that are larger or smaller than your vehicle's original tires
may affect the accuracy of your speedometer and odometer.
Information About Tire Quality Grades
New vehicles are fitted with tires that have their Tire Quality
Grade (described below) molded into the tire's sidewall. These
Tire Quality Grades are determined by standards that the
United States Department of Transportation has set.
Tire Quality Grades apply to new pneumatic tires for use on
this vehicle. They do not apply to deep tread, winter-type snow
tires, space-saver or temporary use spare tires, tires with
nominal rim diameters of 10 to 12 inches (254-305 mm) or
limited production tires as defined in Title 49 Code of Federal
Regulations Part 575.104(c)(2).
U.S. Department of Transportation — Tire quality grades: The
U.S. Department of Transportation requires Ford to give you the
following information about tire grades exactly as the
government has written it.
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.
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