What is Tire Size Aspect Ratio

What is tire size aspect ratio? This is a frequently asked question for people who are confused about the numbers and letters tire size measurements. These measurements are typically written in a certain format on the sidewall of the tire.

The measurements on the sidewall include the type, section width, aspect ratio and diameter. Specialized tires may also have additional information. The numbers are placed into the format SWWW-AA-X-DD NNT, such as P220-30-R16 90H.

The first letter signifies the type of vehicle and/or service that the tire was designed for. A tire with a "P" (for "passenger") means that it is suitable for cars, minivans, SUVs and light trucks. Other tire sizes are ST ("special trailer" for boat/car trailers), LT ("light truck" for large vehicles) and T ("temporary spare").

Section width (WWW) is measured in millimeters and represents the distance between the widest part of the outer sidewall to the widest part of inside sidewall. For this reason, wider tires have high width numbers.

The aspect ratio (AA, also known as the section height) is the percentage of section width that comes from the height from wheel to tread. For example, if the aspect ratio is listed as 30, then the sidewall height is 30% of the section width. The higher the aspect ratio, the taller the sidewall is.

The letter following the aspect ratio (X) is the internal construction indicator. Most tires have an "R" for this letter. The R stands for "radial construction," meaning that the plies of the tire body radiate outward from the center of the wheel. A "D" in this place means that the wheel has diagonal, criss-crossed plies. This is common for spare tires. Belted tires have a "B" in this place and are extremely rare.

The "DD" number is the diameter of the wheel that the tire should be mounted on, expressed in inches.

The final two digit number is the maximum amount of weight (in pounds or  kilograms) that the tire can support. The final letter is the speed rating (charts are available online to show what each letter stands for).

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