255/50R20 vs 275/40R21
275/40R21 has an overall diameter of 753.4 mm (29.66") compared with 763.0 mm (30.04") for 255/50R20, a difference of -9.6 mm (-1.26 %). The sidewall changes by -17.5 mm, section width by +20 mm and ride height by -4.8 mm. Using the ETRTO rolling circumference, 255/50R20 turns 430 times per kilometre and 275/40R21 turns 435 times. With 275/40R21 fitted to a car calibrated for 255/50R20, an indicated 100 km/h is a true 98.7 km/h, and the odometer records 101.3 km for every 100 km driven. This is within the commonly quoted 3 % tolerance.
Full comparison
| Measurement | 255/50R20 | 275/40R21 | Difference |
|---|---|---|---|
| Section width | 255 mm | 275 mm | +20 mm |
| Sidewall height | 127.5 mm | 110.0 mm | -17.5 mm |
| Rim diameter | 20" | 21" | +1.0" |
| Overall diameter | 763.0 mm (30.04") | 753.4 mm (29.66") | -9.6 mm (-1.26 %) |
| Static circumference | 2397 mm | 2367 mm | -30 mm |
| Rolling circumference (ETRTO) | 2327 mm | 2298 mm | -29 mm |
| Revs per km | 430 | 435 | +5 |
| Ride height | -4.8 mm | ||
| Inch equivalent | 30x10R20 | 29.7×10.8R21 | |
| Approved rim widths (approx.) | 7″–9″ | 9″–10.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.5 | 49.4 | 59.2 | 79.0 | 98.7 | 108.6 |
| Error (km/h) | -0.5 | -0.6 | -0.8 | -1.0 | -1.3 | -1.4 |
Assumes the speedo was accurate on 255/50R20. Odometer records 101.3 km for every 100 km actually driven.
- The 275/40R21 is 9.6 mm (1.26 %) smaller in diameter, so your speedo will read high and your odometer will over-record distance.
- Section width changes by +20 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 275/40R21 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -9.6 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 98.7 km/h (over-read 1.3 km/h; ADR 18 allows up to 13.9 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 275/40R21 instead of 255/50R20?
Overall diameter changes by -9.6 mm (-1.26 %). That is within the usual 3 % rule of thumb, so it is generally an acceptable swap provided the load index and speed rating meet your tyre placard and the tyre suits your rim width. 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 275/40R21?
Your speedo will read high by about 1.26 %. At an indicated 100 km/h you would actually be travelling 98.7 km/h, and at 110 km/h about 108.6 km/h.
What is the rolling circumference of 275/40R21?
Static circumference is 2367 mm; the ETRTO rolling circumference under load is about 2298 mm, which is 435 revolutions per kilometre.
275/40R21 in inches
275/40R21 is approximately 29.7×10.8R21: 29.7" tall, 10.8" wide, on a 21" rim, with a 4.3" sidewall.
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