295/40R21 vs 255/55R20
255/55R20 has an overall diameter of 788.5 mm (31.04") compared with 769.4 mm (30.29") for 295/40R21, a difference of +19.1 mm (+2.48 %). The sidewall changes by +22.3 mm, section width by -40 mm and ride height by +9.6 mm. Using the ETRTO rolling circumference, 295/40R21 turns 426 times per kilometre and 255/55R20 turns 416 times. With 255/55R20 fitted to a car calibrated for 295/40R21, an indicated 100 km/h is a true 102.5 km/h, and the odometer records 97.6 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 295/40R21 | 255/55R20 | Difference |
|---|---|---|---|
| Section width | 295 mm | 255 mm | -40 mm |
| Sidewall height | 118.0 mm | 140.3 mm | +22.3 mm |
| Rim diameter | 21" | 20" | -1.0" |
| Overall diameter | 769.4 mm (30.29") | 788.5 mm (31.04") | +19.1 mm (+2.48 %) |
| Static circumference | 2417 mm | 2477 mm | +60 mm |
| Rolling circumference (ETRTO) | 2347 mm | 2405 mm | +58 mm |
| Revs per km | 426 | 416 | -10 |
| Ride height | +9.6 mm | ||
| Inch equivalent | 30.3×11.6R21 | 31x10R20 | |
| Approved rim widths (approx.) | 10″–11.5″ | 7″–9″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 41.0 | 51.2 | 61.5 | 82.0 | 102.5 | 112.7 |
| Error (km/h) | +1.0 | +1.2 | +1.5 | +2.0 | +2.5 | +2.7 |
Assumes the speedo was accurate on 295/40R21. Odometer records 97.6 km for every 100 km actually driven.
- The 255/55R20 is 19.1 mm (2.48 %) larger in diameter, so your speedo will read low and your odometer will under-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 255/55R20 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 255/55R20 is 19.1 mm larger than 295/40R21; 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 102.5 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 2.5 % 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 255/55R20 instead of 295/40R21?
Overall diameter changes by +19.1 mm (+2.48 %). 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 255/55R20?
Your speedo will read low by about 2.48 %. At an indicated 100 km/h you would actually be travelling 102.5 km/h, and at 110 km/h about 112.7 km/h.
What is the rolling circumference of 255/55R20?
Static circumference is 2477 mm; the ETRTO rolling circumference under load is about 2405 mm, which is 416 revolutions per kilometre.
255/55R20 in inches
255/55R20 is approximately 31x10R20: 31.0" tall, 10.0" wide, on a 20" rim, with a 5.5" sidewall.
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