Free tool
Tyre size calculator & conversion tool
Enter the tyre size on your sidewall and the size you are considering. We work out overall diameter, sidewall height, circumference, revolutions per kilometre and how far your speedometer would read out — so you can check a replacement size before you buy.
Current tyre size
225/45R17 — 25" overall diameter, 101.3 mm sidewall
Replacement tyre size
235/40R18 — 25.4" overall diameter, 94 mm sidewall
Side-by-side comparison
| Measurement | 225/45R17 | 235/40R18 | Difference |
|---|---|---|---|
| Sidewall height | 101.3 mm | 94 mm | -7.2 mm |
| Overall diameter | 634.3 mm (25") | 645.2 mm (25.4") | 10.9 mm (1.72%) |
| Circumference | 1992.7 mm | 2027 mm | 34.2 mm |
| Revolutions per km | 501.8 | 493.4 | -8.5 |
| Speedo reads 100 km/h, true speed | 100 km/h | 101.7 km/h | 1.7 km/h |
Overall diameter differs by 1.72%, which is inside the ±3% guideline normally used for a plus-size or alternative fitment. Always confirm load index, speed rating and clearance for your vehicle before ordering.
How to read your tyre size
A size such as 225/45R17 means a 225 mm section width, a sidewall height that is 45% of that width (101.25 mm), and a 17-inch rim. The letters and numbers after the size — for example 94W — are the load index and speed rating, and your replacement tyres should meet or exceed the values on your vehicle's tyre placard.
Metric to inch conversion
Light-truck and 4x4 tyres are often quoted in inches, such as 31x10.5R15. To convert a metric size to overall inches, multiply the rim diameter by 25.4, add twice the sidewall height, then divide by 25.4 — exactly what the calculator above does for you.
Why the ±3% guideline matters
Overall diameter controls speedometer and odometer accuracy, gearing and driver-assistance calibration. Staying within about 3% of the original diameter keeps those systems close to factory behaviour. Also check that your chosen size clears suspension and bodywork at full lock.
Not sure what fits? Search by make, model and year on our home page, browse every stocked size in the size directory, or check remaining tread with the free AI Tread Check.