285/60R18 vs 255/55R20
255/55R20 has an overall diameter of 788.5 mm (31.04") compared with 799.2 mm (31.46") for 285/60R18, a difference of -10.7 mm (-1.34 %). The sidewall changes by -30.7 mm, section width by -30 mm and ride height by -5.4 mm. Using the ETRTO rolling circumference, 285/60R18 turns 410 times per kilometre and 255/55R20 turns 416 times. With 255/55R20 fitted to a car calibrated for 285/60R18, an indicated 100 km/h is a true 98.7 km/h, and the odometer records 101.4 km for every 100 km driven. This is within the commonly quoted 3 % tolerance.
Full comparison
| Measurement | 285/60R18 | 255/55R20 | Difference |
|---|---|---|---|
| Section width | 285 mm | 255 mm | -30 mm |
| Sidewall height | 171.0 mm | 140.3 mm | -30.7 mm |
| Rim diameter | 18" | 20" | +2.0" |
| Overall diameter | 799.2 mm (31.46") | 788.5 mm (31.04") | -10.7 mm (-1.34 %) |
| Static circumference | 2511 mm | 2477 mm | -34 mm |
| Rolling circumference (ETRTO) | 2438 mm | 2405 mm | -33 mm |
| Revs per km | 410 | 416 | +6 |
| Ride height | -5.4 mm | ||
| Inch equivalent | 31.5×11.2R18 | 31x10R20 | |
| Approved rim widths (approx.) | 8″–9.5″ | 7″–9″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.5 | 49.3 | 59.2 | 78.9 | 98.7 | 108.5 |
| Error (km/h) | -0.5 | -0.7 | -0.8 | -1.1 | -1.3 | -1.5 |
Assumes the speedo was accurate on 285/60R18. Odometer records 101.4 km for every 100 km actually driven.
- The 255/55R20 is 10.7 mm (1.34 %) 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 18" to 20", so new wheels are required.
Is 255/55R20 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -10.7 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 98.7 km/h (over-read 1.3 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 255/55R20 instead of 285/60R18?
Overall diameter changes by -10.7 mm (-1.34 %). 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 255/55R20?
Your speedo will read high by about 1.34 %. At an indicated 100 km/h you would actually be travelling 98.7 km/h, and at 110 km/h about 108.5 km/h.
What is the rolling circumference of 255/55R20?
Static circumference is 2477 mm; the ETRTO rolling circumference under load is about 2405 mm, which is 416 revolutions per kilometre.
255/55R20 in inches
255/55R20 is approximately 31x10R20: 31.0" tall, 10.0" wide, on a 20" rim, with a 5.5" sidewall.
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