225/55R17 vs 255/40R18
255/40R18 has an overall diameter of 661.2 mm (26.03") compared with 679.3 mm (26.74") for 225/55R17, a difference of -18.1 mm (-2.66 %). The sidewall changes by -21.7 mm, section width by +30 mm and ride height by -9.0 mm. Using the ETRTO rolling circumference, 225/55R17 turns 483 times per kilometre and 255/40R18 turns 496 times. With 255/40R18 fitted to a car calibrated for 225/55R17, an indicated 100 km/h is a true 97.3 km/h, and the odometer records 102.7 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 225/55R17 | 255/40R18 | Difference |
|---|---|---|---|
| Section width | 225 mm | 255 mm | +30 mm |
| Sidewall height | 123.8 mm | 102.0 mm | -21.7 mm |
| Rim diameter | 17" | 18" | +1.0" |
| Overall diameter | 679.3 mm (26.74") | 661.2 mm (26.03") | -18.1 mm (-2.66 %) |
| Static circumference | 2134 mm | 2077 mm | -57 mm |
| Rolling circumference (ETRTO) | 2072 mm | 2017 mm | -55 mm |
| Revs per km | 483 | 496 | +13 |
| Ride height | -9.0 mm | ||
| Inch equivalent | 26.7×8.9R17 | 26x10R18 | |
| Approved rim widths (approx.) | 6″–8″ | 8.5″–10″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 38.9 | 48.7 | 58.4 | 77.9 | 97.3 | 107.1 |
| Error (km/h) | -1.1 | -1.3 | -1.6 | -2.1 | -2.7 | -2.9 |
Assumes the speedo was accurate on 225/55R17. Odometer records 102.7 km for every 100 km actually driven.
- The 255/40R18 is 18.1 mm (2.66 %) smaller in diameter, so your speedo will read high and your odometer will over-record distance.
- Section width changes by +30 mm. Check guard, strut and inner-arch clearance and that the rim width is within the approved range for the new tyre.
- Rim diameter changes from 17" to 18", so new wheels are required.
Is 255/40R18 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -18.1 mm is within the +15 mm / -26 mm allowance for a passenger car (MA/MB) (measured against the largest size the manufacturer designates for the model). | OK |
| Speedometer (ADR 18/03) A smaller tyre makes the speedo read high: at an indicated 100 km/h you would be doing 97.3 km/h (over-read 2.7 km/h; ADR 18 allows up to 13.7 km/h at that speed). | OK |
| Load index (tyre placard / ADR 42) Enter the placard load index and the replacement tyre load index to check. | ? |
| Speed rating (tyre placard / ADR 42) Enter the placard speed rating and the replacement tyre speed rating to check. | ? |
Frequently asked
Can I fit 255/40R18 instead of 225/55R17?
Overall diameter changes by -18.1 mm (-2.66 %). That is close to the 3 % rule of thumb; it may fit, but check guard, strut and brake clearance and your local rules. In Australia VSB 14 allows up to +15 mm for passenger cars and +50 mm for 4WDs/goods vehicles without certification.
How much will my speedo be out with 255/40R18?
Your speedo will read high by about 2.66 %. At an indicated 100 km/h you would actually be travelling 97.3 km/h, and at 110 km/h about 107.1 km/h.
What is the rolling circumference of 255/40R18?
Static circumference is 2077 mm; the ETRTO rolling circumference under load is about 2017 mm, which is 496 revolutions per kilometre.
255/40R18 in inches
255/40R18 is approximately 26x10R18: 26.0" tall, 10.0" wide, on a 18" rim, with a 4.0" sidewall.
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