285/60R18 vs 275/50R20
275/50R20 has an overall diameter of 783.0 mm (30.83") compared with 799.2 mm (31.46") for 285/60R18, a difference of -16.2 mm (-2.03 %). The sidewall changes by -33.5 mm, section width by -10 mm and ride height by -8.1 mm. Using the ETRTO rolling circumference, 285/60R18 turns 410 times per kilometre and 275/50R20 turns 419 times. With 275/50R20 fitted to a car calibrated for 285/60R18, an indicated 100 km/h is a true 98 km/h, and the odometer records 102.1 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 285/60R18 | 275/50R20 | Difference |
|---|---|---|---|
| Section width | 285 mm | 275 mm | -10 mm |
| Sidewall height | 171.0 mm | 137.5 mm | -33.5 mm |
| Rim diameter | 18" | 20" | +2.0" |
| Overall diameter | 799.2 mm (31.46") | 783.0 mm (30.83") | -16.2 mm (-2.03 %) |
| Static circumference | 2511 mm | 2460 mm | -51 mm |
| Rolling circumference (ETRTO) | 2438 mm | 2388 mm | -49 mm |
| Revs per km | 410 | 419 | +8 |
| Ride height | -8.1 mm | ||
| Inch equivalent | 31.5×11.2R18 | 30.8×10.8R20 | |
| Approved rim widths (approx.) | 8″–9.5″ | 7.5″–9.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.2 | 49.0 | 58.8 | 78.4 | 98.0 | 107.8 |
| Error (km/h) | -0.8 | -1.0 | -1.2 | -1.6 | -2.0 | -2.2 |
Assumes the speedo was accurate on 285/60R18. Odometer records 102.1 km for every 100 km actually driven.
- The 275/50R20 is 16.2 mm (2.03 %) 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 275/50R20 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -16.2 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 98.0 km/h (over-read 2.0 km/h; ADR 18 allows up to 13.8 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 275/50R20 instead of 285/60R18?
Overall diameter changes by -16.2 mm (-2.03 %). 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 275/50R20?
Your speedo will read high by about 2.03 %. At an indicated 100 km/h you would actually be travelling 98 km/h, and at 110 km/h about 107.8 km/h.
What is the rolling circumference of 275/50R20?
Static circumference is 2460 mm; the ETRTO rolling circumference under load is about 2388 mm, which is 419 revolutions per kilometre.
275/50R20 in inches
275/50R20 is approximately 30.8×10.8R20: 30.8" tall, 10.8" wide, on a 20" rim, with a 5.4" sidewall.
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