Reading Tire Size's

• 1). Locate the OE tire size. This is found on the sidewall of the tire and will look similar to "275/70R18."
  
  
• 2). 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.
  
  
• 3). 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.
  
  
• 4). 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.
  
  

Calculating Tire Diameter

• 1). 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).
  
  
• 2). 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.
  
  
• 3). 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.