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READING YOUR TIRE
SPEED RATING
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Service Description
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.
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Use the following chart to determine the maximum load-carrying
capacity based on a tire's load index:
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Speed Rating
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).
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*z rating refers to open ended speed capability.
Warning:
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.
Winter Tires And Speed Ratings
To maintain tire speed capacity indicated on the vehicle placard, winter tires with the same or higher speed rating should be used. If you decide to use winter tires with speed ratings below what is shown on the vehicle placard, the maximum vehicle operating speed must be reduced to the lowest tire speed rating of the tire mounted on the vehicle. In all cases, Yokohama recommends operating speeds not in excess of legal posted speed limits and that speed be reduced appropriately based on weather and road conditions. Winter tire speed ratings do not indicate safe vehicle operating speeds for winter road conditions, including snow, ice, water etc. And drivers must modify vehicular speeds based upon adverse climate changes and conditions.
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