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-carrying capacity is branded on thre tire's sidewall, the load index is used as a quick reference.
P185/60R14This example shows you how to determine the maximum load-carrying capacity based on a tire's load index.
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. For a tire to be speed symbol of "H, Q, V, W, Y" or "Z".
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 (ISO) system 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.
The Tire Industry Safety Council bulletin says: "A reasonable person realizes that driving speeds are dictated by many factors, particularly such things as the weather, road surface and mechanical condition of the vehicle. These tire symbols do not mean that motorists can drive safely at the maximum speed for which the tire is rated or in excess of the posted speed limits."
The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on specified government test course. Tested against an industry standard, the assigned numerical grade indicates how well the tread lasts compared with a reference stadard of 100. A treadwear rating 200 means the tread wears twice as well as the standard. Actual wear depends on the conditions under which the tires is used. Driving habits, service practices, differences in road surface and varying climates all affect treadwear.