…longer wearing tires? In the past, the best a driver could hope for, in regards to treadwear, was an option that lasted approximately 20,000 miles. This was the case for so long that it has become a common misconception that all low profile tires or tires with a high speed rating have rapid…
Service descriptions are required on all speed rated (except for Z-speed rated) tires manufactured since 1991. The first two digits ( 87 S) represent the tire's load index and are followed by a single letter (87 S ) identifying the tire's speed rating.
Understanding Speed Ratings …example, the "Z" in 245/45ZR17 or the "H" in 205/55R16 89H. This letter is the speed rating of the tire. Tires are tested in laboratory conditions under load by tire manufacturers to determine the maximum speed capability of the tire.
Many tire speed rating charts also list the “Z” rating as anything over 149 mph. Some manufacturers will insert a “Z” in the middle of the tire size description (right after the aspect ratio) when the tire is rated at “W” or “Y.”
As manufacturers continue to add speed to their vehicles, tire speed ratings evolve to match the speeds. For example, Z was the highest rated speed at 149+ until W & Y were used to match the higher speeds of exotic sports cars.
Load rating. Tires are specified by the manufacturer with a maximum load rating. Loads exceeding the rating can result in unsafe conditions that can lead to steering instability and even rupture. For a table of load ratings, see tire code. Speed rating. The speed rating denotes the maximum speed at which a tire is designed to be operated.
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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."
A tire’s certified speed rating is given a letter from A to Z, ranging from 5km/h (3mph) to above 300 km/h (186 mph). This rating system (see below) shows the top speed for which a tire is certified.
Athough we don’t recommend it, if you use a tire with a speed rating lower than your vehicle’s original tires, you will reduce your vehicle’s top speed limit to the tires’ speed rating. The handling of a performance vehicle may be different when the replacement tires are not the same speed rating.