Reading a Tire Size
Locate the OE tire size. This is found on the sidewall of the tire and will look similar to "245/65R17."
Note the width of the tire from the first number in the tire size. In this case 245 represents the width across the tire, in millimeters, from the widest point.
Calculate the tire's sidewall height. In the example tire size, 245/65R17, 65 represents the height of the tire's sidewall, from rim to tread, as a percentage of the tire width. In this case the tire's height would be 159.25 millimeters or 6.27 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, 245/65R17, 17 represents the wheel diameter, in inches, that the tire is intended to be mounted on.
Calculating Tire Diameter
Calculate the overall tire height, in inches, using the following formula ((Width/25.4)xSidewall Percentage)x2
A 245/65R17 tire would have an overall tire height of 12.54 inches, ((245/25.4)x65\%)x2.
Combine the overall tire height with the wheel diameter.
Using the example of 245/65R17, you would add 12.54 to 17 for a final diameter of 29.54 inches.
Using the 3 percent variance rule, you would be able to install a new wheel and tire combination that was anywhere from 28.65 inches to 30.43 inches in total diameter.