How to convert metric to inches for tire sizes

Written by gwen chance
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How to convert metric to inches for tire sizes
Centimetres are divided into 10 millimetres. (centimètre image by danimages from Fotolia.com)

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.

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Metric Basics

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.

How to convert metric to inches for tire sizes
Centimetres are divided into 10 millimetres. (centimètre image by danimages from Fotolia.com)

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.

How to convert metric to inches for tire sizes
Wheel rims are often sized in inches. (Speichenrad image by mp focus | painhart from Fotolia.com)

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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