Every tire approved for highway use by the Department of Transportation, including those on your vehicle, comes with a particular speed rating. The speed rating is a letter corresponding to the maximum safe speed at which a tire can be driven, as determined by laboratory testing.
Reading Tire Speed Ratings
Speed ratings can affect the way you drive. To maintain your vehicle's original speed capability, you need to equip it with tires that meet or exceed the original equipment tire's speed rating. Mixing tires of different speed ratings can reduce your maximum safe speed. If you equip your vehicle with tires of different speed ratings, the lowest speed rating will determine your vehicle's maximum speed of which the tires can safely handle. Be aware of your original equipment tire's speed rating to avoid this problem.
Look for a tire's speed rating by checking the end of its size code. Most likely, the last figure in the tire size code will be a letter representing the speed rating. These letters correspond to the tire's maximum safe speed. Letters farther along the alphabet correspond to higher speeds, with "H" being an exception. Find out what your tire's speed rating means using the standardized chart provided:
Unique Speed Ratings
The "Z" rating may appear on any tire rated above 149 MPH. Tires with "Z" ratings were originally thought to be to the highest rated tires a vehicle would require. However, the development of faster vehicles led to the production of two new speed ratings, "W" and "Y." These two ratings provide a better indicator of the tire's capacity for speeds higher than 149 MPH. The "Z," "W," and "Y," ratings appear on tires for sports cars, exotics, or other vehicles engineered for ultra-high speeds.
"H" is the only speed rating that doesn't appear in alphabetical order. A holdover from the early European speed rating system, the tire experts of the time chose not to change the original rating indicated by "H."
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