Tire Size & Speedometer Calculator

Read the tire code before comparing

This tool accepts metric width/aspect/wheel codes, not flotation sizes such as 33x12.50R17. Width is the nominal section width, not tread width. Aspect ratio is sidewall height as a percentage of width; the wheel diameter is in inches. Supported planning bounds are 100-400 mm width, 20-100% aspect ratio, and 10-30 inch wheels. Values inside those bounds do not prove that a real tire exists or fits a vehicle.

Method: nominal geometry and rolling ratio

Sidewall height S = width x aspect / 100. Nominal diameter D = wheel inches x 25.4 + 2S, in mm. Circumference C = pi x D. Let r = replacement D / original D. Diameter change = 100(r - 1)%; nominal ride-height change = (replacement D - original D) / 2.

Assuming the instruments use wheel revolutions, were accurate with the original tire, and are not recalibrated, actual mph = indicated mph x r and actual miles = indicated miles x r. Indicated miles for 100 actual miles = 100 / r. This last output shows the inverse correction explicitly, so it is harder to accidentally apply the adjustment backward.

Worked example: 205/55R16 to 225/45R17

The original sidewall is 112.75 mm and diameter is 631.9 mm. The replacement sidewall is 101.25 mm and diameter is 634.3 mm. Diameter increases by about 0.3798%, nominal ride height by 1.2 mm, and section width by 20 mm. A 60 mph indication corresponds to about 60.2279 mph; a 100 mile indicated trip corresponds to about 100.3798 miles. Conversely, 100 actual miles register about 99.6216 miles.

Interpret the result without treating it as fitment approval

A positive diameter change makes the modeled actual speed higher than indicated. A negative result does the opposite. A width change is not remaining fender clearance: wheel width, offset, suspension position, steering angle, and the specific tire's dimensions all matter. The ride-height output describes nominal radius change only, not a suspension modification.

Do not use a generic percentage threshold as proof of safety. Check the vehicle placard and manufacturer-approved sizes, wheel compatibility, load capacity, speed rating, and clearance with a qualified installer. AWD systems and stability controls can have vehicle-specific restrictions that this arithmetic cannot evaluate.

Assumptions, sources, and limitations

The calculation uses unloaded circular geometry. Real rolling circumference changes with construction, wear, inflation, loading, and instrument calibration; manufacturer revolutions-per-mile data or a measured calibration is better for final corrections. It does not predict fuel economy, certify road legality, or recommend inflation pressure.

For a transparent comparison of the same nominal method, see TireSize.com's tire-code explanation and calculator (undated web reference). NIST SP 811, Appendix B.8 (2008 edition) gives the exact inch conversion: 1 inch = 25.4 mm. The modeled tire geometry is not a measured tire specification.

Related driving decisions

• Gas mileage cost calculator for measured trip consumption after checking distance calibration.
• Cost per mile calculator for ownership spending, not tire fitment.

Last updated: 2026-10-09. Geometry estimate only; confirm the specific vehicle and tire specifications.