255/55R20 vs 265/45R21
265/45R21 has an overall diameter of 771.9 mm (30.39") compared with 788.5 mm (31.04") for 255/55R20, a difference of -16.6 mm (-2.11 %). The sidewall changes by -21.0 mm, section width by +10 mm and ride height by -8.3 mm. Using the ETRTO rolling circumference, 255/55R20 turns 416 times per kilometre and 265/45R21 turns 425 times. With 265/45R21 fitted to a car calibrated for 255/55R20, an indicated 100 km/h is a true 97.9 km/h, and the odometer records 102.2 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 | 265/45R21 | Difference |
|---|---|---|---|
| Section width | 255 mm | 265 mm | +10 mm |
| Sidewall height | 140.3 mm | 119.3 mm | -21.0 mm |
| Rim diameter | 20" | 21" | +1.0" |
| Overall diameter | 788.5 mm (31.04") | 771.9 mm (30.39") | -16.6 mm (-2.11 %) |
| Static circumference | 2477 mm | 2425 mm | -52 mm |
| Rolling circumference (ETRTO) | 2405 mm | 2354 mm | -51 mm |
| Revs per km | 416 | 425 | +9 |
| Ride height | -8.3 mm | ||
| Inch equivalent | 31x10R20 | 30.4×10.4R21 | |
| Approved rim widths (approx.) | 7″–9″ | 8.5″–10″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.2 | 48.9 | 58.7 | 78.3 | 97.9 | 107.7 |
| Error (km/h) | -0.8 | -1.1 | -1.3 | -1.7 | -2.1 | -2.3 |
Assumes the speedo was accurate on 255/55R20. Odometer records 102.2 km for every 100 km actually driven.
- The 265/45R21 is 16.6 mm (2.11 %) 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 265/45R21 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -16.6 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 97.9 km/h (over-read 2.1 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 265/45R21 instead of 255/55R20?
Overall diameter changes by -16.6 mm (-2.11 %). 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 265/45R21?
Your speedo will read high by about 2.11 %. At an indicated 100 km/h you would actually be travelling 97.9 km/h, and at 110 km/h about 107.7 km/h.
What is the rolling circumference of 265/45R21?
Static circumference is 2425 mm; the ETRTO rolling circumference under load is about 2354 mm, which is 425 revolutions per kilometre.
265/45R21 in inches
265/45R21 is approximately 30.4×10.4R21: 30.4" tall, 10.4" wide, on a 21" rim, with a 4.7" sidewall.
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