A tire speed rating defines the maximum speed that a tire may travel at before failing. A brief look at the tires available from tire dealers will quickly reveal that each tire has a distinct speed rating. Each tire manufactured for an automobile is designed to operate at this maximum given speed for up to ten minutes without danger of malfunction.
A tyre's maximum speed limit is set from B-ratings, which allow speeds up to 50 miles per hour, to Y-ratings for high-speed performance exceed 186 miles per hour. Most street legal sports cars come equipped with z-rated tires that are limited to 149 miles per hour.
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Determining Speed Rating
A quick look at the tires on your car will reveal a rather cryptic set of code printed on their sidewalls. This is known as the ISO Metric tire code and appears as a set of numbers and letters and may be imprinted in one of three ways. For example: P205/45ZR17 88W or 205/45ZR17 or 205/45R17 88ZW. It may be worth noting that prior to 1991, tire speed ratings were placed inside the tire size, before the "R" character as in 205/45ZR17. Some tire manufacturers continue to use this format.
Why Use the Tire Code?
Tire codes have been standardised and are regulated by the European Tyre and Rim Technical Organization and the U.S. Tire and Rim Association (T&RA). These associations had been formed to resolve the issue of tires requiring specific rims. These organisations work to unify the manufacturers of tires and rims and set down guidelines for the production of interchangeable tires, wheels, and valves. Speed ratings were instated as vehicles began to outperform tires that were not up to speed with the high performance demands placed upon them. The speed rating system allows the properly rated tire to be matched to the characteristics of the vehicle.
Reading Tire Codes
The following tire code, P205/45R17 88W, states that our example tire is a radial 205mm wide tire with a 90mm high sidewall meant to be mounted on a 17 inch wheel for a passenger vehicle with a maximum load of 560 Kilogram per tire and can sustain speeds up to 168 miles per hour.
The P code represents the vehicle rating. The P denotes that the tire is meant for a Passenger vehicle. Alternately, codes such as "LT" for light truck or "T" for temporary spare tires may be used.
The following set of numbers defines the physical size of the tire. The first number denotes the nominal section width of the tire, which is measured in millimetres from the widest point at both outside edges. The second set of numbers refers to the aspect ratio of the sidewall height as a percentage to the total width of the tire. In other words, the sidewall is 90 millimetres high (45 per cent of the 200 millimetre width of the tire).
In most cases, a single letter is present to denote the type of construction that composes the tire. R stands for Radial, D for Diagonal, and B for Belt Bias type constructions.
The next number denotes the inner diameter of the tire, measured in inches, indicating the size of the wheel that the tire may be installed onto.
The following two digit number represents the Load Index, or the total amount of weight that a single tire can support.
And finally, the last one or two letter combination denotes the speed rating of the tire.
Tire Speed Ratings
Speed ratings vary greatly from tire to tire, based upon carcase construction, process type, and function. Most new vehicles are required by law to be equipped with tires that are rated for higher speeds than the maximum speed of the vehicle. Common speed ratings for tires sold in the United States are: S-rated (maximum of 112mph), V-rated (up to149 mph), and Z-rated (exceeds 149mph). There are two speed ratings reserved for extreme high speed performance, W- and Y-rated tires are capable of sustaining speeds up to 169mph and 186mph, respectively.
It is vital to properly match the tire speed ratings to the performance of your vehicle. Exceeding the maximum speed rating of your tire could result in tire failure or damage, which may put you, and possibly others, in danger. Do not mix tires with different speed ratings, as the tire with the lowest rating will limit the tire-related speed. Consult your vehicle's Owners Manual or a qualified technician if you are uncertain what speed rated tires are required.
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