225/65R17 vs 235/55R18
235/55R18 has an overall diameter of 715.7 mm (28.18") compared with 724.3 mm (28.52") for 225/65R17, a difference of -8.6 mm (-1.19 %). The sidewall changes by -17.0 mm, section width by +10 mm and ride height by -4.3 mm. Using the ETRTO rolling circumference, 225/65R17 turns 453 times per kilometre and 235/55R18 turns 458 times. With 235/55R18 fitted to a car calibrated for 225/65R17, an indicated 100 km/h is a true 98.8 km/h, and the odometer records 101.2 km for every 100 km driven. This is within the commonly quoted 3 % tolerance.
Full comparison
| Measurement | 225/65R17 | 235/55R18 | Difference |
|---|---|---|---|
| Section width | 225 mm | 235 mm | +10 mm |
| Sidewall height | 146.3 mm | 129.3 mm | -17.0 mm |
| Rim diameter | 17" | 18" | +1.0" |
| Overall diameter | 724.3 mm (28.52") | 715.7 mm (28.18") | -8.6 mm (-1.19 %) |
| Static circumference | 2275 mm | 2248 mm | -27 mm |
| Rolling circumference (ETRTO) | 2209 mm | 2183 mm | -26 mm |
| Revs per km | 453 | 458 | +5 |
| Ride height | -4.3 mm | ||
| Inch equivalent | 28.5×8.9R17 | 28.2×9.3R18 | |
| Approved rim widths (approx.) | 6″–7.5″ | 6.5″–8.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.5 | 49.4 | 59.3 | 79.1 | 98.8 | 108.7 |
| Error (km/h) | -0.5 | -0.6 | -0.7 | -0.9 | -1.2 | -1.3 |
Assumes the speedo was accurate on 225/65R17. Odometer records 101.2 km for every 100 km actually driven.
- The 235/55R18 is 8.6 mm (1.19 %) 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.
- Rim diameter changes from 17" to 18", so new wheels are required.
Is 235/55R18 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -8.6 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 98.8 km/h (over-read 1.2 km/h; ADR 18 allows up to 13.9 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 225/65R17?
Overall diameter changes by -8.6 mm (-1.19 %). That is within the usual 3 % rule of thumb, so it is generally an acceptable swap provided the load index and speed rating meet your tyre placard and the tyre suits your rim width. 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 1.19 %. At an indicated 100 km/h you would actually be travelling 98.8 km/h, and at 110 km/h about 108.7 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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