255/55R20 vs 295/40R21
295/40R21 has an overall diameter of 769.4 mm (30.29") compared with 788.5 mm (31.04") for 255/55R20, a difference of -19.1 mm (-2.42 %). The sidewall changes by -22.2 mm, section width by +40 mm and ride height by -9.6 mm. Using the ETRTO rolling circumference, 255/55R20 turns 416 times per kilometre and 295/40R21 turns 426 times. With 295/40R21 fitted to a car calibrated for 255/55R20, an indicated 100 km/h is a true 97.6 km/h, and the odometer records 102.5 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 255/55R20 | 295/40R21 | Difference |
|---|---|---|---|
| Section width | 255 mm | 295 mm | +40 mm |
| Sidewall height | 140.3 mm | 118.0 mm | -22.2 mm |
| Rim diameter | 20" | 21" | +1.0" |
| Overall diameter | 788.5 mm (31.04") | 769.4 mm (30.29") | -19.1 mm (-2.42 %) |
| Static circumference | 2477 mm | 2417 mm | -60 mm |
| Rolling circumference (ETRTO) | 2405 mm | 2347 mm | -58 mm |
| Revs per km | 416 | 426 | +10 |
| Ride height | -9.6 mm | ||
| Inch equivalent | 31x10R20 | 30.3×11.6R21 | |
| Approved rim widths (approx.) | 7″–9″ | 10″–11.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.0 | 48.8 | 58.5 | 78.1 | 97.6 | 107.3 |
| Error (km/h) | -1.0 | -1.2 | -1.5 | -1.9 | -2.4 | -2.7 |
Assumes the speedo was accurate on 255/55R20. Odometer records 102.5 km for every 100 km actually driven.
- The 295/40R21 is 19.1 mm (2.42 %) smaller in diameter, so your speedo will read high and your odometer will over-record distance.
- Section width changes by +40 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 295/40R21 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -19.1 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 97.6 km/h (over-read 2.4 km/h; ADR 18 allows up to 13.8 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 295/40R21 instead of 255/55R20?
Overall diameter changes by -19.1 mm (-2.42 %). 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 295/40R21?
Your speedo will read high by about 2.42 %. At an indicated 100 km/h you would actually be travelling 97.6 km/h, and at 110 km/h about 107.3 km/h.
What is the rolling circumference of 295/40R21?
Static circumference is 2417 mm; the ETRTO rolling circumference under load is about 2347 mm, which is 426 revolutions per kilometre.
295/40R21 in inches
295/40R21 is approximately 30.3×11.6R21: 30.3" tall, 11.6" wide, on a 21" rim, with a 4.6" sidewall.
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