When tire shopping, you become bombarded with numbers that may not mean much to you. One important number on a tire is its size. Tires are sized by width, aspect ratio and rim diameter.The width and aspect ratio measurements are typically measured in millimeters. Typically, this is no problem as your tire size is listed in several places on the vehicle. However, if you own an older car that used tires measured in inches or have purchased new rims, which are measured in inches, you must convert the tire size to inches to find the correct size.
Read the sidewall of the tire and locate the tire size. The size is formatted as XXX/XXRXX, the "X" indicates variable numbers. An example of a common tire size is 225/60R16.
Read the first three digits and multiply them by 0.04 -- the number of inches per mm -- and make a note of this size. This size indicates the tire's width from bead to bead, which indicates the width of rim this tire fits. In the 225/60R16 example, the width is 9 inches (225 X 0.04).
Read the second two numbers on the tire size. This set of numbers is known as the "aspect ratio," which means the sidewall height. This number is relative to the tire's width. In the 225/60R16 example, the sidewall is 60 percent of the tire's width, which calculates to 135 mm.
Multiply the sidewalls calculated height -- in Step 3 -- by 0.04 to get the sidewall's height in inches. The 225/60R16 tire's sidewall height calculates to 5.4 inches.
Combine the two converted numbers from Steps 2 through 4 with the final number on the tire, which is already listed in inches, to show the tire's exact size in inches. The 225/60R16 example calculates to 9/5.4R16.
