Don't let converting between inches and millimetres intimidate you. It's simple multiplication and division for all measurements tires. You can take vintage tire specifications cited in inches and find a modern metric equivalent. Or, you can convert metric measurements to inches when needed to purchase other suspension parts sized in inches.
One inch equals 25.4 millimetres or 2.54 centimetres. Something 5 inches long is 12.7 centimetres or 127 millimetres (5 multiplied by 2.54 and 25.4, respectively). To convert a metric measurement, divide centimetres by 2.54 or the inch equivalent by 25.4 for millimetres to inches. Multiplying your millimetre measurement by .03937 also gives an inch equivalent.
It's the Same for Tires
Tire and wheel manufacturers use a combination of inch and metric measurements, depending on the company's country of origin, historical use and tire part being referenced. Tread and sidewall height are listed in millimetres and rim diameter is often in inches.
Calculators Make it Easy
Use the search features and conversion calculators offered at major tire maker and dealer websites. These sites and calculators make it easy for you to fit tires for your car or to calculate all the tire dimensions.
Figuring Out the Diameter in Inches
Determine tire diameter in inches by calculating it from the metric sidewall size. Multiply the section width by the aspect ratio (first two numbers in the size) and multiply again by 2. Divide by 25.4 to convert the millimetres to inches. Add the rim diameter (last number in the sidewall size and already in inches) for your nominal tire diameter.
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