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If you are changing rim sizes or looking to change tire sizes, the numbers can get confusing. For example, a tire size of 225/60R16 means that the tire is 225 millimeters wide (at the widest point of the sidewall), the aspect ratio (sidewall height) is 60 and the center hole diameter is 16 inches. Only two of the three measurements are needed to fit a tire to a rim: the section width and center hole diameter.
Step 1
Set the rim upright as if it were mounted on a vehicle.
Step 2
Measure the width of the rim from the inside of one bead to the inside of the other bead. The bead is the lip of the rim.
Step 3
Multiply the width in inches by 25.4 (the number of millimeters per inch). This calculation gives you the first number in the tire size. Typically, you can add up to 10 millimeters to the width. So if you had an 8.5-inch rim, the section width range you can safely use is between 215 and 225.
Measure the overall height of the rim and round down for the diameter. So if the rim is 16-1/4 inches tall, the tires center hole diameter would need to be 16 inches. The reason for rounding down is to compensate for the height of the bead.
Tip
- The aspect ratio can be properly gauged only by test-fitting a tire and rim combination on the vehicle.
Items you will need
- Calculator
- Tape measure
