Decode any tyre size code, get the true diameter, circumference and revs per mile, and compare two sizes with the speedometer error between them. Runs entirely in your browser — your numbers never leave this device.
| Measure | A — 205/55R16 | B — 225/45R17 |
|---|---|---|
| Sidewall height | 112.75 mm | 101.25 mm |
| Overall diameter | 631.9 mm | 634.3 mm |
| Overall diameter | 24.88 in | 24.97 in |
| Circumference | 1985.17 mm | 1992.71 mm |
| Revolutions per km | 503.73 | 501.83 |
| Revolutions per mile | 810.68 | 807.61 |
| Speedometer shows | True speed on B |
|---|---|
| 20 | 20.08 |
| 30 | 30.11 |
| 40 | 40.15 |
| 50 | 50.19 |
| 60 | 60.23 |
| 70 | 70.27 |
| 80 | 80.3 |
| 90 | 90.34 |
| 100 | 100.38 |
A common rule of thumb is to keep the overall diameter within about 3% of the original size: beyond that, speedometer and odometer error, gearing and clearance all start to matter. This page does the geometry — fitment for your specific car is a tyre professional's call.
| Code part | What it means |
|---|---|
| 225 | Section width = 225 mm |
| 65 | Sidewall = 65% of width = 146.25 mm |
| R | Radial construction |
| 17 | Rim diameter = 17 in = 431.8 mm |
| Overall | Diameter = 431.8 + 2 × 146.25 = 724.3 mm (28.52 in) |
A tyre code like 225/65R17 is a complete dimension set in disguise. The first number is the section width in millimetres, the second is the sidewall height as a percentage of that width — not a millimetre figure, which is the single most misread part of the code — the R marks radial construction, and the last number is the rim diameter in inches. From those three numbers the whole geometry follows: the sidewall is 225 × 65% = 146.25 mm, the rim contributes 17 × 25.4 = 431.8 mm, and the overall diameter is rim plus two sidewalls, 724.3 mm. Circumference is π times diameter, and revolutions per mile is a mile's length in millimetres divided by that circumference.
Comparing two sizes is where the geometry earns its keep. The speedometer and odometer do not measure speed or distance directly — they count wheel revolutions and multiply by the circumference of the tyre the car was calibrated for. Fit a tyre of a different diameter and every reading scales by the diameter ratio: a tyre 0.41% smaller makes the car travel 59.75 when the dial shows 60, and it also spins more revolutions per mile, which is why gearing feels shorter. This page prints the diameter difference as a percentage, both revs-per-mile figures, and a speedometer table from 20 to 100 so the error is a number, not a surprise at a speed check.
The widely used guidance is to stay within about 3% of the original overall diameter when changing sizes: beyond that, speedometer error, effective gearing, ABS and stability-system assumptions and physical clearance all move at once. This calculator does the geometry exactly and nothing more — load ratings, speed ratings and whether a size physically fits your car and rims are fitment questions for the tyre maker's data and a fitter, and no diameter match makes an under-rated tyre safe.
It is the aspect ratio: the sidewall height expressed as a percentage of the section width. On a 225/65R17 the sidewall is 65% of 225 mm, which is 146.25 mm — it is not itself a measurement in millimetres, and reading it as one is the most common decoding mistake.
The speedometer counts wheel revolutions and assumes the calibrated tyre's circumference. True speed scales with the diameter ratio: fit a tyre 10.84% larger in diameter (225/65R17 to 265/70R17) and an indicated 60 is a true 66.50. The table on this page gives the true speed for every indicated 20–100.
The common guidance is within about 3% of the original overall diameter. Inside that band, speedometer error stays small (under about 2 units at 60) and gearing is barely affected. The 205/55R16 to 225/45R17 swap, for example, differs by only +0.38%. Beyond 3%, check clearance, gearing and system calibration with a fitter.
They are the link between the wheel and everything that counts it: speedometer and odometer calibration, and engine RPM through the gearing — the Gear Ratio Calculator starts from the same tyre diameter. A smaller tyre records more revs per mile (810.68 for a 205/55R16 against 707.26 for a 225/65R17), so the odometer over-reads distance travelled.
Diameter is only one dimension. Rim diameter must match exactly, and width, load index and speed rating must suit the car — none of which a diameter calculation can judge. Use this page for the geometry and the speedometer consequences, and the tyre manufacturer's fitment data for the rest.
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