Calculating the diameter of the original equipment (OE) tires on a vehicle provides a way to select new wheels and tires of the correct size. Ensuring you have a proper size is essential to your safety and for a correct speed reading. When installing new wheels and/or tires, most professionals suggest being within a three percent, positive or negative, variance of the OE.
Locate the OE tire size. This is found on the sidewall of the tire and will look similar to “275/70R18."
Note the width of the tire from the first number in the tire size. In this case, "275" represents the width across the tire, in millimeters, from the widest point.
Calculate the tires sidewall height.
In the example tire size, 275/70R18, "70" represents the height of the tire’s sidewall, from rim to tread, as a percentage of the the tire width. In this case, the tire’s height would be 192.5 millimeters or 7.58 inches. This is also known as the tire’s series.
Locate the OE wheel diameter; this is the last number in the tire size. In the example, 275/70R18, "18" represents the wheel diameter, in inches, that the tire is intended to be mounted on.
Calculate the overall tire height, in inches, using the following formula ((Width/25.4)xSidewall Percentage)x2
A 275/70R18 tire would have an overall tire height of 15.16 inches, ((275/25.4)x70%)x2).
Combine the overall tire height with the wheel diameter.
Using the example of 275/70R18, you would add 15.16 to 18 for a final diameter of 33.16 inches.
Using the three percent variance rule, you would be able to install a new tire and wheel combination that was between 32.16 inches to 34.16 inches in total diameter.
