255/55R18 vs 235/55R18
235/55R18 has an overall diameter of 715.7 mm (28.18") compared with 737.7 mm (29.04") for 255/55R18, a difference of -22.0 mm (-2.98 %). The sidewall changes by -11.0 mm, section width by -20 mm and ride height by -11.0 mm. Using the ETRTO rolling circumference, 255/55R18 turns 444 times per kilometre and 235/55R18 turns 458 times. With 235/55R18 fitted to a car calibrated for 255/55R18, an indicated 100 km/h is a true 97 km/h, and the odometer records 103.1 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 255/55R18 | 235/55R18 | Difference |
|---|---|---|---|
| Section width | 255 mm | 235 mm | -20 mm |
| Sidewall height | 140.3 mm | 129.3 mm | -11.0 mm |
| Rim diameter | 18" | 18" | 0.0" |
| Overall diameter | 737.7 mm (29.04") | 715.7 mm (28.18") | -22.0 mm (-2.98 %) |
| Static circumference | 2318 mm | 2248 mm | -69 mm |
| Rolling circumference (ETRTO) | 2250 mm | 2183 mm | -67 mm |
| Revs per km | 444 | 458 | +14 |
| Ride height | -11.0 mm | ||
| Inch equivalent | 29x10R18 | 28.2×9.3R18 | |
| Approved rim widths (approx.) | 7″–9″ | 6.5″–8.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 38.8 | 48.5 | 58.2 | 77.6 | 97.0 | 106.7 |
| Error (km/h) | -1.2 | -1.5 | -1.8 | -2.4 | -3.0 | -3.3 |
Assumes the speedo was accurate on 255/55R18. Odometer records 103.1 km for every 100 km actually driven.
- The 235/55R18 is 22 mm (2.98 %) smaller in diameter, so your speedo will read high and your odometer will over-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.
Is 235/55R18 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -22.0 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 97.0 km/h (over-read 3.0 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 235/55R18 instead of 255/55R18?
Overall diameter changes by -22.0 mm (-2.98 %). 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 235/55R18?
Your speedo will read high by about 2.98 %. At an indicated 100 km/h you would actually be travelling 97 km/h, and at 110 km/h about 106.7 km/h.
What is the rolling circumference of 235/55R18?
Static circumference is 2248 mm; the ETRTO rolling circumference under load is about 2183 mm, which is 458 revolutions per kilometre.
235/55R18 in inches
235/55R18 is approximately 28.2×9.3R18: 28.2" tall, 9.3" wide, on a 18" rim, with a 5.1" sidewall.
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