275/50R20 vs 285/60R18
285/60R18 has an overall diameter of 799.2 mm (31.46") compared with 783.0 mm (30.83") for 275/50R20, a difference of +16.2 mm (+2.07 %). The sidewall changes by +33.5 mm, section width by +10 mm and ride height by +8.1 mm. Using the ETRTO rolling circumference, 275/50R20 turns 419 times per kilometre and 285/60R18 turns 410 times. With 285/60R18 fitted to a car calibrated for 275/50R20, an indicated 100 km/h is a true 102.1 km/h, and the odometer records 98 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 275/50R20 | 285/60R18 | Difference |
|---|---|---|---|
| Section width | 275 mm | 285 mm | +10 mm |
| Sidewall height | 137.5 mm | 171.0 mm | +33.5 mm |
| Rim diameter | 20" | 18" | -2.0" |
| Overall diameter | 783.0 mm (30.83") | 799.2 mm (31.46") | +16.2 mm (+2.07 %) |
| Static circumference | 2460 mm | 2511 mm | +51 mm |
| Rolling circumference (ETRTO) | 2388 mm | 2438 mm | +49 mm |
| Revs per km | 419 | 410 | -8 |
| Ride height | +8.1 mm | ||
| Inch equivalent | 30.8×10.8R20 | 31.5×11.2R18 | |
| Approved rim widths (approx.) | 7.5″–9.5″ | 8″–9.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 40.8 | 51.0 | 61.2 | 81.7 | 102.1 | 112.3 |
| Error (km/h) | +0.8 | +1.0 | +1.2 | +1.7 | +2.1 | +2.3 |
Assumes the speedo was accurate on 275/50R20. Odometer records 98.0 km for every 100 km actually driven.
- The 285/60R18 is 16.2 mm (2.07 %) larger in diameter, so your speedo will read low and your odometer will under-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 20" to 18", so new wheels are required.
Is 285/60R18 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 285/60R18 is 16.2 mm larger than 275/50R20; the limit without certification for a passenger car (MA/MB) is +15 mm. Larger increases need an approved signatory / engineering certificate (modification code LS or state equivalent) and may not be permitted at all. | Cert. |
| Speedometer (ADR 18/03) A larger tyre makes the speedo read lower: at an indicated 100 km/h you would be doing 102.1 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 2.1 % increase is usually absorbed. Verify with a GPS speed app after fitting. | Verify |
| 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 285/60R18 instead of 275/50R20?
Overall diameter changes by +16.2 mm (+2.07 %). 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 285/60R18?
Your speedo will read low by about 2.07 %. At an indicated 100 km/h you would actually be travelling 102.1 km/h, and at 110 km/h about 112.3 km/h.
What is the rolling circumference of 285/60R18?
Static circumference is 2511 mm; the ETRTO rolling circumference under load is about 2438 mm, which is 410 revolutions per kilometre.
285/60R18 in inches
285/60R18 is approximately 31.5×11.2R18: 31.5" tall, 11.2" wide, on a 18" rim, with a 6.7" sidewall.
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