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Tire size converter

Turn a metric size like 265/70R17 into inches, convert an inch size like 33×12.50R15 to metric, and compare two sizes side by side.

Free, no sign-upUpdated September 2026
Your numbers
Current size
New size
Convert an inch size (optional)

Diameter difference

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CurrentNew
Speedometer reads 60, you are going
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Inch size as metric
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The 3% rule

Staying within about 3% of the original diameter usually keeps the speedometer, ABS and gearing close to stock. Bigger changes are fine, but plan for recalibration or regearing.

Reading a tire size

Diameter (in) = width (mm) × aspect ratio ÷ 100 × 2 ÷ 25.4 + wheel diameter

In 265/70R17, 265 is the section width in millimeters, 70 means the sidewall is 70% as tall as the tire is wide, and 17 is the wheel diameter in inches. There are two sidewalls, so the sidewall height counts twice.

Inch sizes like 33×12.50R15 list the overall diameter, the section width and the wheel. The converter works out the equivalent metric width and aspect ratio. For the new size's effect on rpm and regearing, use the gear ratio, rpm & speedometer calculator.