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Tire speed rating is extremely important to handling and braking, yet it may be one of the most misunderstood tire ratings.

For many, the first introduction to speed rating is in buying tires. The tire salesperson may ask, is that "H" or "V" rated? Unfortunately, many tire salespeople have little understanding of what the rating means other than one cost more than the other. It’s "just" the speed rating, is a very poor and incomplete answer.

Speed rating is listed on the sidewall of the tire, following the size

When an engineer designs the suspension for a vehicle, the tires are a major consideration. Many factors are considered before a tire is specified. One major factor, relating to handling and braking is centrifugal distortion. This is the amount the tire distorts when subjected to speed, braking or handling. With more belting and the better quality, there is less distortion. This is important, as the design of the suspension and brakes partially determine the centrifugal load on the tires.

Centrifugal force distorts tires as they spin or corner

The method used to decide resistance to centrifugal distortion is a speed test, and the rating is known as a speed rating. Tires are tested by spinning them at speed, until they fail. The greater the speed of failure the higher the rating.

Speed of failure for different tire rating

The lowest rated tires, usually used on automobiles, are "S" rated. This rating means the tire is safe for 112 MPH. The next rating is "T" and then "U" and so on. The better the tire is constructed, the higher the speed of failure. This is also the source of many misunderstandings. The speed rating does not imply how fast the vehicle will be driven; it expresses the characteristics of the tire.

Speed rating relates to distortion of the tire under a load and not just how fast the vehicle is driven

Tire distortion causes lack of handling and braking

For instance, when the steering is turned, on a vehicle traveling at speed, centrifugal force distorts the tire, in the direction of travel. Friction of the contact patch determines control of the vehicle. The more belting in the tire, the less distortion and the better the vehicle will handle. Similar forces also act on tires during braking.

You never know when an emergency will arise

Under normal conditions, most tires can withstand loads placed on them by the suspension. In an emergency maneuver, the proper tire is necessary to withstand the increased load.

Speed rating required is listed on the placard and in the owner's manual

Handling and braking is extremely important to most people. Engineering that goes into the vehicle largely determines this. Engineers specify the speed rating required for good reason and place the information on the vehicle and in the owner’s manual.

Never install a tire, rated lower than the vehicle calls for

Placing a higher rated tire, than is required, is not necessary but will generally increase the handling and braking of many vehicles. Substituting an underrated tire, may save a few dollars. It may also cost the driver far more than can ever be saved.

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