Many tires come with a service description added onto 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 the tire's sidewall, the load index is used as a quick reference.
Use the following chart 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 rated, it must meet certain minimum industry standards for reaching and sustaining that specified speed. Domestically, high-performance tires typically are speed rated. Yokohama defines high-performance tires with a speed symbol of 'H, V, W, Y, (Y)' or 'Z'. Speed symbols may currently be marked on a tire in any of three ways: 245/40ZR18; 245/40ZR18 97Y; or 245/40R18 97Y. The International Standards 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 (illustrated in the second and third examples above).
If tires with different speed ratings are mounted on the same vehicle, the tire with the lowest rating will dictate safe maximum vehicle speed.
*This refers to tires that have a maximum speed capability above 240 KM/H (149 mph). 'Z' may appear in the size designation. Tires that have a maximum speed capability above 300 KM/H (186 mph) must have a 'Z' appear in the size designation. Consult the tire manufacturer for maximum speed when there is no service description.
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.
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.