Tyre size calculator
What you have on the car now, and what you are thinking about. Everything else follows from those six numbers.
Showing 225/45R17 → 235/45R17. Change either row and every figure below re-reads.
The verdict
Inside the ±3% overall-diameter band vehicle makers calibrate to. Speedometer, ABS and stability control all stay inside the error they were validated for.
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·both tyres drawn to one scale from their size designations; the ±3% ring is the industry fitment convention, not a legal limit·REVIEWED ETRTO STANDARD
- Diameter change
- +9.0mm
- Ride height
- +4.5mm
- Speedo at 100
- 101.4km/h true
- Odometer / 1,000 km
- 1014km real
Indicated against true
| Speedo reads | You are actually doing | Error | In mph |
|---|---|---|---|
| 50 km/h | 50.7 km/h | +0.7 km/h | 31 shown → 31.5 true |
| 60 km/h | 60.9 km/h | +0.9 km/h | 37 shown → 37.8 true |
| 80 km/h | 81.1 km/h | +1.1 km/h | 50 shown → 50.4 true |
| 90 km/h | 91.3 km/h | +1.3 km/h | 56 shown → 56.7 true |
| 100 km/h | 101.4 km/h | +1.4 km/h | 62 shown → 63.0 true |
| 110 km/h | 111.6 km/h | +1.6 km/h | 68 shown → 69.3 true |
The taller tyre covers more ground per revolution, so the instrument counts fewer turns than it expects and the needle reads low. Every figure in the middle column is faster than what the dial is telling you.
SOURCE·Computed from rolling-diameter ratio·instrument assumes the original circumference·REVIEWED ETRTO STANDARD
How the figures are derived
- Read the size as the specification it is: section width in millimetres, aspect ratio as a percentage of that width, rim diameter in inches.
- Sidewall height = width × aspect ÷ 100. This is the number the size string is really about, and it is the one nobody quotes.
- Overall diameter = rim × 25.4 + two sidewalls — one at the top of the wheel and one at the bottom.
- Circumference = π × overall diameter, which gives revolutions per kilometre and per mile directly.
- Nothing above is measured, estimated, averaged or dated. It is the definition of the designation, so it does not go stale and it has no source to cite beyond the standard itself.
sidewall (mm) = width × aspect ÷ 100 overall diameter (mm) = rim × 25.4 + 2 × sidewall speedo true speed = indicated × (new diameter ÷ old diameter)
The full method, the ±3% rule, and what geometry cannot tell you →
Read this before you buy anything
This calculator is complete about rolling diameter and silent about everything else, because everything else is genuinely not knowable from a size string. Before you commit: check the tyre placard in your driver's door aperture for the sizes, load index and speed symbol your car actually specifies; check the rim width your wheels are, against the range the new size is approved for; and check clearance at full lock with a fitter, on the car, not on a website.
Using the calculator
How do I know if a tyre size will fit my car?
Geometry answers half of it: stay within ±3% of your original rolling diameter and the speedometer, ABS and stability control remain inside the band they were calibrated against. The other half — rim width, arch clearance, load index — is on the placard in your driver's door aperture and in a fitter's hands, not in a size string.
What does ±3% actually refer to?
Overall rolling diameter, measured against the size the vehicle was calibrated for. It is an industry convention rather than a legal limit; what makes it meaningful is that vehicle manufacturers validate their electronics inside it.
Does this calculator work without JavaScript?
The drawing, the verdict and every figure on this page are rendered by the server for the default comparison, so the page is complete and correct with scripting off. The selectors then re-read it live — they change what is displayed, never what is true.