The size of a tire provides information about the wheel it is sized for, the width of the tire, and its proportions. The height or diameter of a tire is not directly included in the size. Tire diameter can be important if you are changing tire size and want to make sure the new size will fit in the wheel space of the vehicle and not rub or bind.

Locate the tire size on the sidewall of the tire and write it down. The tire size will be in a form that looks like this: 245/65R17. There may be additional letters before or after the size, but this information is the size descriptor for the tire.

Calculate the section height of the tire by multiplying the section width times the aspect ratio. The section width is the first three digits of the tire size in millimeters. The aspect ratio is the two digits after the slash before the R. In the example above, the width of 245 millimeters would be multiplied by the aspect ratio of 65 percent, resulting in a section height of 159.25 millimeters.

Multiply the section height times two and convert to inches using a multiplication factor of 0.03937. For the example tire size, the multiplication would be 159.25 times 2 times 0.03937 giving a tire height of 12.54 inches. This height is the measurement of the tire above and below the rim.

Add the tire height amount to the wheel rim diameter. The wheel size is listed after the R in the tire size. In the example the tire goes on a 17inch rim. Adding the tire height to the rim diameter gives a total tire diameter of 29.54 inches.
Tips & Warnings
 The tire diameter calculated would be for a tire inflated to recommended pressure not carrying a load.
References
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