Reading Tire Size

• 1). Locate the OE tire size. This is found on the sidewall of the tire and will look similar to 235/45R18.
  
  
• 2). Note the width of the tire from the first number in the tire size. In the example from the previous step 235 represents the width across the tire, in millimeters, from the widest point.
  
  
• 3). Calculate the tires sidewall height. In the example tire size, 235/45R18, 45 represents the height of the tire's sidewall, from rim to tread, as a percentage of the the tire width. This is also known as the tire's series. In the case of our example the tire's height would be 105.75 millimeters.
  
  
• 4). Locate the OE wheel diameter. This is the last number in the tire size. In the example, 235/45R18, 18 represents the wheel diameter, in inches, that the tire is intended to be mounted on.
  
  

Calculating Tire Diameter

• 1). Calculate the overall tire height, in inches, using the following formula ((width/25.4) x sidewall percentage) x 2
  
  
• 2). A 235/45R18 tire would have an overall tire height of 8.33 inches. ((235/25.4) x 45 percent) x 2).
  
  
• 3). Combine the overall tire height with the wheel diameter. Using the example of 235/45R18 you would add 8.33 to 18 for a final tire diameter of 23.33 inches.
  
  
• 4). Using the 3 percent variance rule, you would be able to change to a tire that was between 22.63 and 24.03 inches in total diameter.