Tool
Motorcycle Tire Size Calculator
A stamp such as 180/55-17 is width in millimeters, sidewall height as a percent of that width, and rim diameter in inches. The calculator estimates overall diameter and circumference from those three numbers. It is not a brand-specific loaded measurement.
Estimated circumference
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When to use this calculator
Use it when you are stepping a rear tire up or down a size, and you want the direction of the circumference change. A taller tire raises true road speed relative to a speedometer that was calibrated on the shorter tire.
The geometry ignores tread brand, construction, load, and pressure. If you can tape the mounted tire through the centerline, enter that circumference on the speed-in-gear page instead.
This is not a pressure recommendation and not a VIN specification. The only inputs are the three numbers on the sidewall, plus an optional second size.
Zero width, zero aspect, or zero rim produces a field error.
Worked sizes
Sidewall 99 mm, diameter 629.8 mm, circumference about 77.90 inches. That is the default on the speed-in-gear page.
The extra 10 mm of width raises the sidewall and the circumference. The second-size note should show a positive percent. Does your speedometer then read a little slow?
A lower aspect shortens the sidewall and the circumference. Road speed at a given indicated MPH would be lower if the instrument was set to the taller tire.
Technical questions
How is diameter calculated?
Sidewall millimeters = width × aspect ÷ 100. Diameter millimeters = rim inches × 25.4 + 2 × sidewall. Circumference = π × diameter.
Why can two 180/55-17 tires roll differently?
Carcass shape, wear, and pressure change the real radius. The stamp is a nominal size, not a measured rollout.
Does a wider tire always raise the speedometer error?
On this geometric model, a wider tire at the same aspect and rim is taller, so circumference rises. A wider tire with a lower aspect can go the other way. Enter both stamps and read the percent.