245/45R18 vs 255/40R18
255/40R18 has an overall diameter of 661.2 mm (26.03") compared with 677.7 mm (26.68") for 245/45R18, a difference of -16.5 mm (-2.43 %). The sidewall changes by -8.2 mm, section width by +10 mm and ride height by -8.2 mm. Using the ETRTO rolling circumference, 245/45R18 turns 484 times per kilometre and 255/40R18 turns 496 times. With 255/40R18 fitted to a car calibrated for 245/45R18, an indicated 100 km/h is a true 97.6 km/h, and the odometer records 102.5 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 245/45R18 | 255/40R18 | Difference |
|---|---|---|---|
| Section width | 245 mm | 255 mm | +10 mm |
| Sidewall height | 110.3 mm | 102.0 mm | -8.2 mm |
| Rim diameter | 18" | 18" | 0.0" |
| Overall diameter | 677.7 mm (26.68") | 661.2 mm (26.03") | -16.5 mm (-2.43 %) |
| Static circumference | 2129 mm | 2077 mm | -52 mm |
| Rolling circumference (ETRTO) | 2067 mm | 2017 mm | -50 mm |
| Revs per km | 484 | 496 | +12 |
| Ride height | -8.2 mm | ||
| Inch equivalent | 26.7×9.6R18 | 26x10R18 | |
| Approved rim widths (approx.) | 7.5″–9″ | 8.5″–10″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.0 | 48.8 | 58.5 | 78.1 | 97.6 | 107.3 |
| Error (km/h) | -1.0 | -1.2 | -1.5 | -1.9 | -2.4 | -2.7 |
Assumes the speedo was accurate on 245/45R18. Odometer records 102.5 km for every 100 km actually driven.
- The 255/40R18 is 16.5 mm (2.43 %) smaller in diameter, so your speedo will read high and your odometer will over-record distance.
- Section width changes by +10 mm. Check guard, strut and inner-arch clearance and that the rim width is within the approved range for the new tyre.
Is 255/40R18 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -16.5 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 97.6 km/h (over-read 2.4 km/h; ADR 18 allows up to 13.8 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 245/45R18?
Overall diameter changes by -16.5 mm (-2.43 %). 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 2.43 %. At an indicated 100 km/h you would actually be travelling 97.6 km/h, and at 110 km/h about 107.3 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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