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. Because the maximum tire load capacity is branded on the tire's
sidewall, the load index is used as a quick reference.
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. Yokohama goes one step further and defines 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 (illustrated in the second and third examples above).
*z rating refers to open ended speed capability.