Speed ratings are important for highways without speed limits, such as the German Autobahn. The minimum speed rating, represented by the letter L, is 75 mph, but it is only used for light truck and off-road tires. The rating of most spare tires, N, is 87 mph, well above the standard speed limits on U.S. highways.
The speed rating may also influence a tire's ability to help your car achieve peak performance. If your car's manual requires a certain speed rating for the tires, make sure the rating of the ones you buy at least matches the minimum rating to assure optimal performance.
Find the service numbers on the sidewall of the tires. The service numbers are located between the tire's name and the tire manufacturer's name.
Locate the three characters that represent the speed rating and load index of the tires. There will be three characters grouped together, two digits and one letter, in the middle of the service numbers. The letter will be the speed rating of the tire. For example, if the three letters were 91U, U would be the speed rating.
Use a speed rating chart, available online, to determine the maximum speed rating from the rating on the tire. For example, if "U" was the speed rating, the maximum speed would be 124 mph.
