285/40R21 vs 275/50R20
275/50R20 has an overall diameter of 783.0 mm (30.83") compared with 761.4 mm (29.98") for 285/40R21, a difference of +21.6 mm (+2.84 %). The sidewall changes by +23.5 mm, section width by -10 mm and ride height by +10.8 mm. Using the ETRTO rolling circumference, 285/40R21 turns 431 times per kilometre and 275/50R20 turns 419 times. With 275/50R20 fitted to a car calibrated for 285/40R21, an indicated 100 km/h is a true 102.8 km/h, and the odometer records 97.2 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 285/40R21 | 275/50R20 | Difference |
|---|---|---|---|
| Section width | 285 mm | 275 mm | -10 mm |
| Sidewall height | 114.0 mm | 137.5 mm | +23.5 mm |
| Rim diameter | 21" | 20" | -1.0" |
| Overall diameter | 761.4 mm (29.98") | 783.0 mm (30.83") | +21.6 mm (+2.84 %) |
| Static circumference | 2392 mm | 2460 mm | +68 mm |
| Rolling circumference (ETRTO) | 2322 mm | 2388 mm | +66 mm |
| Revs per km | 431 | 419 | -12 |
| Ride height | +10.8 mm | ||
| Inch equivalent | 30×11.2R21 | 30.8×10.8R20 | |
| Approved rim widths (approx.) | 9.5″–11″ | 7.5″–9.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 41.1 | 51.4 | 61.7 | 82.3 | 102.8 | 113.1 |
| Error (km/h) | +1.1 | +1.4 | +1.7 | +2.3 | +2.8 | +3.1 |
Assumes the speedo was accurate on 285/40R21. Odometer records 97.2 km for every 100 km actually driven.
- The 275/50R20 is 21.6 mm (2.84 %) 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 21" to 20", so new wheels are required.
Is 275/50R20 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 275/50R20 is 21.6 mm larger than 285/40R21; 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 102.8 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 2.8 % 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 275/50R20 instead of 285/40R21?
Overall diameter changes by +21.6 mm (+2.84 %). 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 low by about 2.84 %. At an indicated 100 km/h you would actually be travelling 102.8 km/h, and at 110 km/h about 113.1 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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