295/40R21 vs 255/50R20
255/50R20 has an overall diameter of 763.0 mm (30.04") compared with 769.4 mm (30.29") for 295/40R21, a difference of -6.4 mm (-0.83 %). The sidewall changes by +9.5 mm, section width by -40 mm and ride height by -3.2 mm. Using the ETRTO rolling circumference, 295/40R21 turns 426 times per kilometre and 255/50R20 turns 430 times. With 255/50R20 fitted to a car calibrated for 295/40R21, an indicated 100 km/h is a true 99.2 km/h, and the odometer records 100.8 km for every 100 km driven. This is within the commonly quoted 3 % tolerance.
Full comparison
| Measurement | 295/40R21 | 255/50R20 | Difference |
|---|---|---|---|
| Section width | 295 mm | 255 mm | -40 mm |
| Sidewall height | 118.0 mm | 127.5 mm | +9.5 mm |
| Rim diameter | 21" | 20" | -1.0" |
| Overall diameter | 769.4 mm (30.29") | 763.0 mm (30.04") | -6.4 mm (-0.83 %) |
| Static circumference | 2417 mm | 2397 mm | -20 mm |
| Rolling circumference (ETRTO) | 2347 mm | 2327 mm | -20 mm |
| Revs per km | 426 | 430 | +4 |
| Ride height | -3.2 mm | ||
| Inch equivalent | 30.3×11.6R21 | 30x10R20 | |
| Approved rim widths (approx.) | 10″–11.5″ | 7″–9″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.7 | 49.6 | 59.5 | 79.3 | 99.2 | 109.1 |
| Error (km/h) | -0.3 | -0.4 | -0.5 | -0.7 | -0.8 | -0.9 |
Assumes the speedo was accurate on 295/40R21. Odometer records 100.8 km for every 100 km actually driven.
- The 255/50R20 is 6.4 mm (0.83 %) 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 21" to 20", so new wheels are required.
Is 255/50R20 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -6.4 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 99.2 km/h (over-read 0.8 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 255/50R20 instead of 295/40R21?
Overall diameter changes by -6.4 mm (-0.83 %). 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 255/50R20?
Your speedo will read high by about 0.83 %. At an indicated 100 km/h you would actually be travelling 99.2 km/h, and at 110 km/h about 109.1 km/h.
What is the rolling circumference of 255/50R20?
Static circumference is 2397 mm; the ETRTO rolling circumference under load is about 2327 mm, which is 430 revolutions per kilometre.
255/50R20 in inches
255/50R20 is approximately 30x10R20: 30.0" tall, 10.0" wide, on a 20" rim, with a 5.0" sidewall.
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