Tire Size Calculator

Compare two tire sizes side by side and see how a change affects diameter, clearance and your speedometer.

Like 225/65R17 vs 245/60R18.
Original Tire
New Tire
Fractions and decimals both work.
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% Diameter Change
Diameter
Width
Sidewall
Circumference
Revs per mile
Actual speed
Ride height change
Speedometer

Tire Comparisons

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How to Compare Tire Sizes

A metric tire size like 225/65R17 gives the section width in millimeters (225), the sidewall height as a percentage of the width (65%), and the wheel diameter in inches (17). The overall diameter is the wheel plus two sidewalls.

Sidewall (in) = width (mm) × aspect ÷ 100 ÷ 25.4
Diameter (in) = wheel + 2 × sidewall   Circumference = π × diameter   Revs per mile = 63,360 ÷ circumference
Actual speed = speedometer reading × new diameter ÷ original diameter

A taller tire covers more ground per turn, so the speedometer (calibrated to the original tire) reads low: you're going faster than it shows.

Worked Example

Tips

Most tire and vehicle makers suggest staying within about 3% of the original diameter to keep the speedometer, ABS and stability control accurate and avoid rubbing. Wider tires may need a different wheel width; check the tire's approved rim width range. Actual tire dimensions vary slightly by brand and tread depth.