Calculating from inches to millimetres can be done using conversion formulas. When measuring your tire, always be precise with the inches measured, as it will make a difference in the tyre's metric size. For most brands, you will see the measurements posted on the tire as three numbers. The first number is the metric section width and it will be listed as a numeral, such as "255." The next number is separated with a forward slash and is the tyre's aspect ratio, such as "45." The final number will be separated by an "R" and is the wheel's diameter in inches. So the entire number on a tire may read: 255/45R17.
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Things you need
- Tape measure
Measure the width of the tire going from front to back in inches. Multiply this number by 25.4 to get the millimetres. This is the first number in the metric number reading and is referred to as the section width.
Measure the height of the tyre's sidewall in inches. Multiply this number by 25.4 and divide the number by the section width. This is referred to as the aspect ratio and is the second set of numbers.
Measure the wheel's overall diameter from top to bottom. Leave this number in inches. This is the final piece of the tire dimension listing.
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