On metric-sized tires, the overall diameter is not listed. This measurement does have an effect on the vehicle in some instances. Placing a tire with a larger diameter on your vehicle can cause the speedometer to read inaccurately, as the tire travels further on a single revolution. Another time the tire's diameter can impact a vehicle is when you are attempting to fit a larger tire beneath a car. If the overall diameter is too large, the tire might rub against the vehicle's body. The overall diameter of a tire can be calculated in only a few steps.
Read the tire size that is stamped on the sidewall. It will read XXX/XXRXX, with the "X" being a variable. A common tire size to use as an example is 205/60R15. The first number is the tires's section width, the second is the aspect ratio or sidewall height and the third is the rim diameter.
Convert the vehicle's aspect ratio to a percentage, this is easily done as the number indicates a percentage. In the example 205/60R15, the aspect ratio is 60 percent. This means the sidewall height is 60 percent of the section width.
Determine your tire's sidewall height, in millimeters, by multiplying the section width by the aspect percentage found in Step 2. In the example tire, the aspect ratio is 123 mm (205 X 0.60).
Convert the section height into inches by multiplying the aspect ratio measurement in Step 3 by 0.04. In the example tire, the aspect ratio converts to 4.92 inches (123 X 0.04).
Multiply the converted aspect ratio, in Step 4, by two. This compensates for the sidewall height at the top and the sidewall height at the bottom of the tire. The example tire measures 9.84 inches.
Add the number calculated in Step 5 to the rim diameter, listed on the sidewall, to come up with the tires overall diameter. The example 205/60R15 tire's overall diameter converts to 24.84 inches.
