Tire Size Converter & Speedo Error Calculator
Convert a metric tyre size into its real overall diameter and circumference, and compare a new size against your current one to see exactly how far the speedometer will read out.
Why circumference matters
The rolling circumference of the rear tyre is the last link in the driveline — it is what actually turns wheel revolutions into distance. Change the aspect ratio or width and that circumference moves, quietly re-gearing the bike and throwing off a speedometer that was calibrated for the original tyre. A couple of millimetres of sidewall is easy to overlook and adds up across a whole revolution.
The circumference this tool gives feeds straight into the Gear Speed Calculator, and a rear-sprocket change in the Chain & Sprocket Ratio Calculator stacks on top of any tyre change.
Frequently Asked Questions
What do the numbers in a tyre size mean?
In 180/55-17, the 180 is the section width in millimetres, the 55 is the aspect ratio (sidewall height as a percentage of width, so 55 percent of 180 = 99 mm), and the 17 is the rim diameter in inches. Overall diameter is the rim (converted to mm) plus two sidewalls, and circumference is π times that diameter.
How does a different tyre change my speedometer?
The speedometer counts wheel revolutions and assumes the original rolling circumference. Fit a tyre with a larger circumference and each revolution covers more ground than the speedo thinks, so it under-reads — you are actually going faster than it shows. A smaller tyre makes it over-read. The calculator gives the exact percentage and what your true speed is when the dash shows 100.
Is a taller tyre from a plus-size safe to fit?
Changing profile affects more than the speedo: overall gearing, ground clearance, handling, and clearance to the swingarm and hugger all move. Small aspect-ratio changes are common, but always stay within the sizes your bike and rim width are rated for, and check physical clearance. Use this tool to understand the speedo and gearing effect before committing.
Why is my calculated diameter slightly different from the real tyre?
The size code describes nominal, unloaded geometry, but manufacturers vary a little around it and a loaded, rolling tyre sits slightly smaller than its free diameter. Expect the calculated figure to be within a percent or two of a tape measure across the actual fitted tyre — close enough for gearing and speedo work.
Educational estimate from nominal tyre geometry. Always fit sizes your bike and rim are rated for, and check physical clearance before changing profile.