Tire Sizing Guide
Tire sizing affects the handling and steering on a car. Since tires are specifically matched to carry a vehicle's weight, choosing the incorrect size can greatly reduce the handling capability of your tires. Cars have specific tire sizing requirements based on vehicle weight, spacing within the wheel well and appropriate fit on the tire rim.
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Significance
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Tire sizing was standardized to aid both consumers and mechanics in installing the correct tires on today's cars. Many cars have computerized systems that measure speed data that is directly affected by tire sizing. Tire sizing is branded onto the side of the tire with a code that provides everything you need to know to purchase the correct size tires for your vehicle. This same information is listed on the door-end label on the driver's side of the car.
Vehicle Codes
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The stamping on the sidewall of the tire provides information on the tire's speed rating, load capacity and the tire's purpose. Tire codes list a series of numbers and letters to explain the dimensions and specifications of a tire. For example, the following represents a typical tire code: P225/50R16 91S.
The "P" stands for "Pmetric," a standardized tire definition in the United States. Tires within the "P" category are passenger vehicles, light trucks, some SUVs and vehicles no greater than a quarter to a half ton. European or metric-sized tires don't have the "P" listed at the beginning of the tire code.
The letter "T" in place of the "P" refers to the temporary spare tire. "LT" means the tire is intended for use on a light truck ranging in weight from three-quarters to a ton, and also pickups, minivans and SUVs. "C" indicates a commercial vehicle such as a delivery van or truck. The letters "ST" stamped on the sidewall of a tire indicate "Special Trailer" service. "ST" tires are used only for boat, car or utility trailers. -
Tire Dimensions
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Using our tire code example (P225/50R16 91S), the first set of numbers after the letter describes the measurement of the widest point on the sidewall of the tire. The "/50" is the measurement of the tire's profile from rim to tread. This measurement is also called sidewall height.
The "R" designates a radial tire. Radial tires have plies of the tire radiate from the center of the tire. A "D" designation indicates diagonal plies to the tire that is often used for spares and on light trucks. A "B" refers to a belted tire. Belted tires are rarely used on cars today.
Tire Diameter
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The tire diameter measure is given next in our example (P225/50R16 91S). The "16" shown after the "R" reflects the measurement of the diameter of the tire and wheel rim. This number differs greatly between makes and models of vehicles.
Tire ratings
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The final two designations on the tire code refer to the load and speed ratings. Both indicate the maximum endurance of the tire under normal conditions. Load rating is a measurement of how much weight the tires can handle both vertically and horizontally. Speed rating indicates the highest speed the tire can travel within normal conditions. In our example, the rating is "S," which lists a rating of 112 mph, a typical speed rating for passenger vehicles. A complete list of speed ratings can be found at the link below.
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Resources
- Photo Credit National Highway Traffic Safety Administration
