Automobile Tire Size Explained
Tire sizing is done with a standardized alpha numeric code, called P-Metric, that describes different aspects of the tire. Our example shows this tire size to be P235/45/R17 94H. Each part of this tire code explains a different characteristic and/or measurement of the tire.
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Prefixes ( P or LT )
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The first part of the tire size is the service type. This prefix indicates if this tire is designed to be a passenger car tire (indicated by P) or a light truck tire (indicated by LT). While a passenger car tire can be used on a light truck, the light truck tire is designed to provide extra load capacity that the passenger car tire can't. Our example of P235/45/R17 is designed to be used on a passenger car.
A tire with the prefix of T is for temporary spare tire use only, and not meant to be a service replacement.
A tire with no prefix at all is called a euro metric tire. Used in Europe and on some suvs, its dimensions are similar, but the load capacity differs slightly.
Section Width
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The first group of numbers in our example tire is the section width. The section width is defined as the cross-sectional width of the tire, measured at the widest part of the sidewall, when installed on a specific size wheel. Measured in millimeters, this measurement is not the tire tread width, as some would suggest, but the width of the sidewall. So, our example of 235 is 235 millimeters wide at the widest point on the sidewall when installed.
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Section Height
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The second set of numbers is two numbers representing the aspect ratio, or section height. Also measured in millimeters, it represents the distance from the rim to the tread as a percentage of section width. On our example tire, the section height is 45. This indicates that this tire is 45 percent of the section width tall. If our section width is 235 mm, our aspect ratio works out to be 105.75 mm tall.
Tire Construction Type
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The letter designation R in our 235/45/R17 indicates this tire is of radial construction, meaning the belt plies radiate outward from the center of the rim. This is the most common type of tire construction and is sold as standard equipment on passenger cars and light trucks in the U.S. This type of construction offers superior wet and dry weather handling, and fuel mileage characteristics.
Any other designation indicates a bias ply tire.
Rim Diameter.
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The number 17 in our 235/45/R17 is the rim diameter that the tire fits. A tire should only be installed on the rim diameter for which it is designed.
This really comes into play when tires are installed. With enough determination, a 16-inch tire can be installed on a 16.5 inch rim. This stresses the tire bead ring and this tire will fail on the road.
Service Description
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The last of the alpha numeric coding is called the service description. It indicates the load index and speed rating.
For more information on speed rating and load indexes, visit www.tirerack.com.
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