Tire Size Calculator

Compare two tire sizes (e.g. 225/45R17) to find the diameter difference and speedometer calibration error.

Free to use · Instant results

Calculator Inputs

Tire 1 Width

Original tire section width, in millimeters (e.g. 225 in 225/45R17).

Tire 1 Aspect Ratio

Original tire sidewall aspect ratio (e.g. 45 in 225/45R17).

Tire 1 Rim Diameter

Original wheel rim diameter, in inches (e.g. 17 in 225/45R17).

Tire 2 Width

Replacement tire section width, in millimeters.

Tire 2 Aspect Ratio

Replacement tire sidewall aspect ratio.

Tire 2 Rim Diameter

Replacement wheel rim diameter, in inches.

Diameter Difference: 1.72%
Speedometer Calibration Error: 1.72%
Tire 1 Overall Diameter: 634.30
Tire 2 Overall Diameter: 645.20

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How It's Calculated

Formula

\text{sidewall} = \text{width} \times \frac{\text{aspect}}{100} \\ \text{diameter} = (\text{rim}_{in} \times 25.4) + 2 \times \text{sidewall} \\ \Delta\% = \frac{\text{diameter}_2 - \text{diameter}_1}{\text{diameter}_1} \times 100

This calculator compares two metric tire sizes by computing each tire's overall diameter: sidewall height (width times aspect ratio divided by 100) plus rim diameter, converted to millimeters. The diameter difference is the percentage change from tire 1 (treated as the original/reference tire) to tire 2 (the replacement). Because a speedometer calibrated for tire 1 infers speed from wheel rotation rate, and distance per rotation scales with diameter, the resulting speedometer calibration error is numerically the same percentage: a larger replacement tire means the speedometer under-reads actual speed, and a smaller replacement tire means it over-reads.

Worked Examples

225/45R17 vs 235/40R18

Tire 1 sidewall = 225 x 45 / 100 = 101.25 mm
Tire 1 diameter = (17 x 25.4) + 2 x 101.25 = 634.3 mm
Tire 2 sidewall = 235 x 40 / 100 = 94 mm
Tire 2 diameter = (18 x 25.4) + 2 x 94 = 645.2 mm
Difference = (645.2 - 634.3) / 634.3 x 100 ≈ 1.72%

Identical tires: 225/45R17 vs 225/45R17

Both diameters = 634.3 mm
Difference = (634.3 - 634.3) / 634.3 x 100 = 0%

Frequently Asked Questions

Why are the diameter difference and speedometer error the same number?

Both figures come from the same ratio: overall diameter of tire 2 divided by overall diameter of tire 1. Circumference (and therefore distance traveled per wheel rotation) is directly proportional to diameter, so the percentage change in diameter is the same as the percentage error a speedometer calibrated for the original tire will show.

Does a positive result mean my speedometer reads too high or too low?

A positive value means tire 2 is larger, so at any given displayed speed your actual speed is higher than shown — the speedometer under-reads. A negative value means tire 2 is smaller, so the speedometer over-reads.

Can I type a size like "225/45R17" directly?

Not in this version — enter the width, aspect ratio, and rim diameter as separate numbers for each tire instead of a combined size string.