Automotive

Tire Size Calculator

Compare two metric tire sizes for diameter, circumference, revolutions per mile and speedometer difference.

Free online toolWorks in your browserNo sign-up required
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How to use this tire size calculator

Enter the requested values above and select Calculate. Change any input to compare scenarios.

Formula and method

Sidewall height = tire width × aspect ratio. Diameter = rim diameter + 2 × sidewall height, after converting millimeters to inches.

Example

Compare 225/65R17 with 235/60R18 to see the diameter difference and approximate indicated-versus-actual speed change.

Important assumptions

A tire that physically fits may still be unsuitable because of load rating, wheel width, suspension clearance, gearing, ABS/traction-control behavior or manufacturer restrictions.

Frequently asked questions

What does the Tire Size Calculator calculate?

Compare two metric tire sizes for diameter, circumference, revolutions per mile and speedometer difference.

How does the calculation work?

Sidewall height = tire width × aspect ratio. Diameter = rim diameter + 2 × sidewall height, after converting millimeters to inches.

Is this calculator free?

Yes. ERICH Calculators is free to use and no account is required.

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Method, assumptions & corrections

See how formulas, sources and limitations are handled across ERICH Calculators.