Many tires come with a service description
added on to the end of the tire's size. These service descriptions
contain a two-digit number (load index) and a letter (speed
rating). The load index is a representation of the maximum load
each tire is designed to support.
Speed ratings are certified maximum sustained
speed designations assigned to passenger car radials and high performance
tires. Because of the evolution of high-speed passenger car travel,
it was necessary to establish a way to rate a tire's high-speed capability.
In the U.S., these ratings are based on tire testing in laboratory
conditions under simulated loads (European testing uses actual road
testing). For a tire to be speed rated by the U.S. Government, it
must meet certain minimum government standards for reaching and sustaining
that specified speed. Domestically, high performance tires must be
speed rated. The tire industry defines high performance tires as those
with speed symbols of "S" or greater and aspect ratios of 70 or lower.
Most tire manufacturer's define high performance tires
with a speed symbol of "H, V, W, Y," or "Z" and an aspect ratio of
70 or less (typically, 60 or less). Conventional passenger car radials
need only meet the minimum Department of Transportation standard of
85 mph. Speed symbols may currently be marked on a tire in any of
three ways: 205/60ZR15; 205/60ZR15 89W; or 205/60R15 89W. The International
Standard Organization system (ISO) currently serves as a worldwide
standard for tire markings. At the end of a transition period, any
speed symbol denoting a fixed maximum speed capability will be at
the end of the service description following the tire marking.
Speed ratings apply only to the tire,
not to the vehicle. Putting a speed rated tire on any car does not
mean the car can be operated at the tire's rated speed.