275/65R18 vs 285/50R20
285/50R20 has an overall diameter of 793.0 mm (31.22") compared with 814.7 mm (32.07") for 275/65R18, a difference of -21.7 mm (-2.66 %). The sidewall changes by -36.2 mm, section width by +10 mm and ride height by -10.9 mm. Using the ETRTO rolling circumference, 275/65R18 turns 402 times per kilometre and 285/50R20 turns 413 times. With 285/50R20 fitted to a car calibrated for 275/65R18, an indicated 100 km/h is a true 97.3 km/h, and the odometer records 102.7 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 275/65R18 | 285/50R20 | Difference |
|---|---|---|---|
| Section width | 275 mm | 285 mm | +10 mm |
| Sidewall height | 178.8 mm | 142.5 mm | -36.2 mm |
| Rim diameter | 18" | 20" | +2.0" |
| Overall diameter | 814.7 mm (32.07") | 793.0 mm (31.22") | -21.7 mm (-2.66 %) |
| Static circumference | 2559 mm | 2491 mm | -68 mm |
| Rolling circumference (ETRTO) | 2485 mm | 2419 mm | -66 mm |
| Revs per km | 402 | 413 | +11 |
| Ride height | -10.9 mm | ||
| Inch equivalent | 32.1×10.8R18 | 31.2×11.2R20 | |
| Approved rim widths (approx.) | 7.5″–9″ | 8″–10″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 38.9 | 48.7 | 58.4 | 77.9 | 97.3 | 107.1 |
| Error (km/h) | -1.1 | -1.3 | -1.6 | -2.1 | -2.7 | -2.9 |
Assumes the speedo was accurate on 275/65R18. Odometer records 102.7 km for every 100 km actually driven.
- The 285/50R20 is 21.7 mm (2.66 %) 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 18" to 20", so new wheels are required.
Is 285/50R20 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -21.7 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 97.3 km/h (over-read 2.7 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 285/50R20 instead of 275/65R18?
Overall diameter changes by -21.7 mm (-2.66 %). 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 285/50R20?
Your speedo will read high by about 2.66 %. At an indicated 100 km/h you would actually be travelling 97.3 km/h, and at 110 km/h about 107.1 km/h.
What is the rolling circumference of 285/50R20?
Static circumference is 2491 mm; the ETRTO rolling circumference under load is about 2419 mm, which is 413 revolutions per kilometre.
285/50R20 in inches
285/50R20 is approximately 31.2×11.2R20: 31.2" tall, 11.2" wide, on a 20" rim, with a 5.6" sidewall.
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