275/50R20 vs 285/40R21
285/40R21 has an overall diameter of 761.4 mm (29.98") compared with 783.0 mm (30.83") for 275/50R20, a difference of -21.6 mm (-2.76 %). The sidewall changes by -23.5 mm, section width by +10 mm and ride height by -10.8 mm. Using the ETRTO rolling circumference, 275/50R20 turns 419 times per kilometre and 285/40R21 turns 431 times. With 285/40R21 fitted to a car calibrated for 275/50R20, an indicated 100 km/h is a true 97.2 km/h, and the odometer records 102.8 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 | 285/40R21 | Difference |
|---|---|---|---|
| Section width | 275 mm | 285 mm | +10 mm |
| Sidewall height | 137.5 mm | 114.0 mm | -23.5 mm |
| Rim diameter | 20" | 21" | +1.0" |
| Overall diameter | 783.0 mm (30.83") | 761.4 mm (29.98") | -21.6 mm (-2.76 %) |
| Static circumference | 2460 mm | 2392 mm | -68 mm |
| Rolling circumference (ETRTO) | 2388 mm | 2322 mm | -66 mm |
| Revs per km | 419 | 431 | +12 |
| Ride height | -10.8 mm | ||
| Inch equivalent | 30.8×10.8R20 | 30×11.2R21 | |
| Approved rim widths (approx.) | 7.5″–9.5″ | 9.5″–11″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 38.9 | 48.6 | 58.3 | 77.8 | 97.2 | 107.0 |
| Error (km/h) | -1.1 | -1.4 | -1.7 | -2.2 | -2.8 | -3.0 |
Assumes the speedo was accurate on 275/50R20. Odometer records 102.8 km for every 100 km actually driven.
- The 285/40R21 is 21.6 mm (2.76 %) 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 20" to 21", so new wheels are required.
Is 285/40R21 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -21.6 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 97.2 km/h (over-read 2.8 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 285/40R21 instead of 275/50R20?
Overall diameter changes by -21.6 mm (-2.76 %). 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 285/40R21?
Your speedo will read high by about 2.76 %. At an indicated 100 km/h you would actually be travelling 97.2 km/h, and at 110 km/h about 107 km/h.
What is the rolling circumference of 285/40R21?
Static circumference is 2392 mm; the ETRTO rolling circumference under load is about 2322 mm, which is 431 revolutions per kilometre.
285/40R21 in inches
285/40R21 is approximately 30×11.2R21: 30.0" tall, 11.2" wide, on a 21" rim, with a 4.5" sidewall.
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