285/50R20 vs 275/65R18
275/65R18 has an overall diameter of 814.7 mm (32.07") compared with 793.0 mm (31.22") for 285/50R20, a difference of +21.7 mm (+2.74 %). The sidewall changes by +36.3 mm, section width by -10 mm and ride height by +10.9 mm. Using the ETRTO rolling circumference, 285/50R20 turns 413 times per kilometre and 275/65R18 turns 402 times. With 275/65R18 fitted to a car calibrated for 285/50R20, an indicated 100 km/h is a true 102.7 km/h, and the odometer records 97.3 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 285/50R20 | 275/65R18 | Difference |
|---|---|---|---|
| Section width | 285 mm | 275 mm | -10 mm |
| Sidewall height | 142.5 mm | 178.8 mm | +36.3 mm |
| Rim diameter | 20" | 18" | -2.0" |
| Overall diameter | 793.0 mm (31.22") | 814.7 mm (32.07") | +21.7 mm (+2.74 %) |
| Static circumference | 2491 mm | 2559 mm | +68 mm |
| Rolling circumference (ETRTO) | 2419 mm | 2485 mm | +66 mm |
| Revs per km | 413 | 402 | -11 |
| Ride height | +10.9 mm | ||
| Inch equivalent | 31.2×11.2R20 | 32.1×10.8R18 | |
| Approved rim widths (approx.) | 8″–10″ | 7.5″–9″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 41.1 | 51.4 | 61.6 | 82.2 | 102.7 | 113.0 |
| Error (km/h) | +1.1 | +1.4 | +1.6 | +2.2 | +2.7 | +3.0 |
Assumes the speedo was accurate on 285/50R20. Odometer records 97.3 km for every 100 km actually driven.
- The 275/65R18 is 21.7 mm (2.74 %) 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 275/65R18 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 275/65R18 is 21.7 mm larger than 285/50R20; 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 102.7 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 2.7 % 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 275/65R18 instead of 285/50R20?
Overall diameter changes by +21.7 mm (+2.74 %). 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 275/65R18?
Your speedo will read low by about 2.74 %. At an indicated 100 km/h you would actually be travelling 102.7 km/h, and at 110 km/h about 113 km/h.
What is the rolling circumference of 275/65R18?
Static circumference is 2559 mm; the ETRTO rolling circumference under load is about 2485 mm, which is 402 revolutions per kilometre.
275/65R18 in inches
275/65R18 is approximately 32.1×10.8R18: 32.1" tall, 10.8" wide, on a 18" rim, with a 7.0" sidewall.
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