The speed rating for a tire is included in the code written on the sidewall of each tire. The recommended speed rating for tires for a specific vehicle can be found on a placard inside the glove compartment, door jamb, or fuel-filler door, according to Consumer Reports.
The speed rating is listed on the sidewall of a car tire as part of the service description code. It is separated by a space from the main part of the code. The speed rating is denoted by a letter or letters listed directly after the load index. The lowest speed rating is N, which denotes 87 miles per hour, as reported by Cars.com.
Speed ratings for tires are used primarily to indicate the speed that can be sustained over a long period of time without the tire sustaining significant damage. The rating is also a predictor of the overall performance of a tire. A higher rating indicates that the tire has additional components, such as extra belts. These components add stability to the tires, according to About.com.
When putting tires on a vehicle, it is important to choose those rated at least as high as those recommended by the vehicle manufacturer, according to Consumer Reports.