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Tire Size Decoded

Tire Size Decoded

Tire size: also printed on the sidewall, the tire size provides information about the tire’s intended purpose, dimensions, load capacity and high temperature/high speed durability. For explanation, we’ll use P235/55R18 99T

P235/55R18 99T: the letter that the tire size begins with specifies the type of vehicle and/or the type of service for which the tires were designed.

P” signifies passenger vehicles, including cars, minivans, sport utility vehicles and light duty pickup trucks.

T” signifies that the tire is a “temporary spare” designed to be used only temporarily until a flat tire can be repaired or replaced (commonly known as a donut).

LT” signifies the tire is a light truck-metric size designed to be used on vehicles capable of carrying heavy cargo or towing large trailers, including medium and heavy-duty pickup trucks, sport utility vehicles and full-size vans.

C” signifies the tire is a commercial tire designed to be used on vans or delivery trucks that are capable of carrying heavy loads.

ST” signifies the tire is a special trailer service size designed only to be used on boat, car or utility trailers.

*If a tire does not begin with a letter, it signifies the tire is a metric size primarily used on European cars, but also vans and sport utility vehicles.

P235/55R18 99T: the three-digit number following the letter(s) identifies the tire’s section width, or cross section, in millimeters, from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel.

P235/55R18 99T: the two digit number following the section width identifies the tire’s profile or aspect ratio. For example, 55 indicates that the tire size’s sidewall height from rim to tread is 55% of its section width.

P235/55R18 99T: this letter identifies the tire’s internal construction.

            “R” indicates the tire has a radial construction

            “D” indicates the tire has a diagonal construction

            “B” indicates the tire has a belted or bias construction

P235/55R18 99T: the number following the tire construction identifies the tire and wheel diameter designed to be matched together. Most tires have a rim diameter expressed in inches (called “inch rim” sizes), but tire/wheel diameters can also be expressed in half inches or millimeters. These unique types can be found mostly on heavy-duty trailers and light trucks, box vans and Michelin® TRX tires.

P235/55R18 99T: finally, the last combination of numbers and letters represents the tire’s service description, identifying the tire’s load index and speed rating. When replacing your tires it’s important to match the load index and speed rating of the original equipment tires to insure safe handling and load carrying capabilities.

Tire Speed Rating Chart

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