How to Figure Tire Sizes

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The evolution of tire specifications has resulted in a size designation that is a combination of letters, metric and English measurements, and mathematical systems. The majority of tires, on passenger vehicles today, utilize this unusual combination to denote their size on the sidewall. Although the system looks strange, it does provide a way to figure a tires size, dimensions and purpose. For this article, we will be using a variation of a 225/40R18 tire size.

  • Locate the letter designation found before the 225/40R18 tire size, it is possible that the tire may have no preceding letter. If the tire size has no preceding letter it is considered a "Metric" size and is commonly used on European vehicles, vans and SUVs. Tires with a "P" are designated for passenger vehicle use, "T" denotes a temporary spare, "LT" represents light truck use and "ST" signifies special trailer service.

  • Proceed to the three numbers following the letter designation. In our example, 225/40R18, this would be the "225." This represents the widest point of the tire in millimeters. To convert the width from millimeters to inches, divide by 25.4. 225mm divided by 25.4 equals 8.86 inches.

  • Note the two number following the tires width. In the example, 225/40R18, this would be "40". The 40 represents the height of the tires sidewall as a percentage of the tires width. In the case of our example, the height of the sidewall in 40 percent of 225mm, 90mm. This could also be represented in inches by taking 40 percent of 8.86 inches.

  • Locate the letter following the two numbers from the previous step, in the case of our example it is an "R". This letter represents the internal construction of the tire. Almost all tires will have the "R" designation, which represents a radial construction. Tire sizes with a "D" denote a bias ply diagonal construction and a "B" represents a belted tire.

    This letter may be proceeded and/or preceded by another letter. Using a variation of our example, 225/40ZR18, the "Z" would represent the speed rating of the tire and the "R" would still represent a radial construction. In the event the tire size was 225/40RF18, the "F" denotes a self-supporting run flat tire.

  • Read the last one or two numbers of the tire size. For a 225/40R18 tire, the last two numbers are "18". This indicates the diameter of the rim, in inches, the tire is intended to be mounted on.

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