What this calculator does
Tire size & speedometer difference works out overall diameter, circumference and speedometer error between two tire sizes. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaSidewall height = width(mm) × aspect ratio/100; diameter = 2×sidewall + rim(in)×25.4; speedometer error % = (new diameter/old diameter − 1) × 100
The inputs explained
| Field | What to enter |
|---|---|
| Current tire width (mm) | A number, measured in mm. Starts at 225. |
| Current aspect ratio (%) | A number, measured in %. Starts at 45. |
| Current rim diameter (in) | A number, measured in in. Starts at 18. |
| New tire width (mm) | A number, measured in mm. Starts at 235. |
| New aspect ratio (%) | A number, measured in %. Starts at 40. |
| New rim diameter (in) | A number, measured in in. Starts at 19. |
| Speedometer reading to check (km/h) | A number, measured in km/h. Starts at 100. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with current tire width
Every other input is held at the calculator’s starting values while current tire width varies. Select any row to load that scenario into the calculator.
How the answer changes with current tire width
| Current tire width (mm) | Diameter change | Old tire diameter | New tire diameter |
|---|---|---|---|
| 113 | 20.0% | 558.9 mm (22.00 in) | 670.6 mm (26.40 in) |
| 169 | 10.1% | 609.3 mm (23.99 in) | 670.6 mm (26.40 in) |
| 225 | 1.65% | 659.7 mm (25.97 in) | 670.6 mm (26.40 in) |
| 338 | -11.9% | 761.4 mm (29.98 in) | 670.6 mm (26.40 in) |
| 450 | -22.2% | 862.2 mm (33.94 in) | 670.6 mm (26.40 in) |
| 675 | -37.0% | 1,064.7 mm (41.92 in) | 670.6 mm (26.40 in) |