235/55R18 vs 255/55R18
255/55R18 has an overall diameter of 737.7 mm (29.04") compared with 715.7 mm (28.18") for 235/55R18, a difference of +22.0 mm (+3.07 %). The sidewall changes by +11.0 mm, section width by +20 mm and ride height by +11.0 mm. Using the ETRTO rolling circumference, 235/55R18 turns 458 times per kilometre and 255/55R18 turns 444 times. With 255/55R18 fitted to a car calibrated for 235/55R18, an indicated 100 km/h is a true 103.1 km/h, and the odometer records 97 km for every 100 km driven. This exceeds the commonly quoted 3 % tolerance; check clearances, ABS/ESC calibration and local roadworthy rules (VSB 14 in Australia).
Full comparison
| Measurement | 235/55R18 | 255/55R18 | Difference |
|---|---|---|---|
| Section width | 235 mm | 255 mm | +20 mm |
| Sidewall height | 129.3 mm | 140.3 mm | +11.0 mm |
| Rim diameter | 18" | 18" | 0.0" |
| Overall diameter | 715.7 mm (28.18") | 737.7 mm (29.04") | +22.0 mm (+3.07 %) |
| Static circumference | 2248 mm | 2318 mm | +69 mm |
| Rolling circumference (ETRTO) | 2183 mm | 2250 mm | +67 mm |
| Revs per km | 458 | 444 | -14 |
| Ride height | +11.0 mm | ||
| Inch equivalent | 28.2×9.3R18 | 29x10R18 | |
| Approved rim widths (approx.) | 6.5″–8.5″ | 7″–9″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 41.2 | 51.5 | 61.8 | 82.5 | 103.1 | 113.4 |
| Error (km/h) | +1.2 | +1.5 | +1.8 | +2.5 | +3.1 | +3.4 |
Assumes the speedo was accurate on 235/55R18. Odometer records 97.0 km for every 100 km actually driven.
- The 255/55R18 is 22 mm (3.07 %) larger in diameter, so your speedo will read low and your odometer will under-record distance.
- Section width changes by +20 mm. Check guard, strut and inner-arch clearance and that the rim width is within the approved range for the new tyre.
- A diameter change over 3 % is outside the usual rule of thumb; check local roadworthy rules (in Australia see VSB 14 / NCOP Section LS) and ABS/ESC calibration.
Is 255/55R18 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 255/55R18 is 22.0 mm larger than 235/55R18; the limit without certification for a passenger car (MA/MB) is +15 mm. Larger increases need an approved signatory / engineering certificate (modification code LS or state equivalent) and may not be permitted at all. | Cert. |
| Speedometer (ADR 18/03) A larger tyre makes the speedo read low: at an indicated 100 km/h you would be doing 103.1 km/h. ADR 18 requires the speedo never to read below true speed; a 3.1 % increase is unlikely to be absorbed by the factory over-read, so the speedo will need recalibration (correction module) and certification. | Cert. |
| 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 255/55R18 instead of 235/55R18?
Overall diameter changes by +22.0 mm (+3.07 %). That is outside the 3 % rule of thumb, so it may rub, upset ABS/ESC calibration and may not be legal without certification. 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 255/55R18?
Your speedo will read low by about 3.07 %. At an indicated 100 km/h you would actually be travelling 103.1 km/h, and at 110 km/h about 113.4 km/h.
What is the rolling circumference of 255/55R18?
Static circumference is 2318 mm; the ETRTO rolling circumference under load is about 2250 mm, which is 444 revolutions per kilometre.
255/55R18 in inches
255/55R18 is approximately 29x10R18: 29.0" tall, 10.0" wide, on a 18" rim, with a 5.5" sidewall.
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