255/40R19 vs 245/40R20
245/40R20 has an overall diameter of 704.0 mm (27.72") compared with 686.6 mm (27.03") for 255/40R19, a difference of +17.4 mm (+2.53 %). The sidewall changes by -4.0 mm, section width by -10 mm and ride height by +8.7 mm. Using the ETRTO rolling circumference, 255/40R19 turns 478 times per kilometre and 245/40R20 turns 466 times. With 245/40R20 fitted to a car calibrated for 255/40R19, an indicated 100 km/h is a true 102.5 km/h, and the odometer records 97.5 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 255/40R19 | 245/40R20 | Difference |
|---|---|---|---|
| Section width | 255 mm | 245 mm | -10 mm |
| Sidewall height | 102.0 mm | 98.0 mm | -4.0 mm |
| Rim diameter | 19" | 20" | +1.0" |
| Overall diameter | 686.6 mm (27.03") | 704.0 mm (27.72") | +17.4 mm (+2.53 %) |
| Static circumference | 2157 mm | 2212 mm | +55 mm |
| Rolling circumference (ETRTO) | 2094 mm | 2147 mm | +53 mm |
| Revs per km | 478 | 466 | -12 |
| Ride height | +8.7 mm | ||
| Inch equivalent | 27x10R19 | 27.7×9.6R20 | |
| Approved rim widths (approx.) | 8.5″–10″ | 8″–9.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 41.0 | 51.3 | 61.5 | 82.0 | 102.5 | 112.8 |
| Error (km/h) | +1.0 | +1.3 | +1.5 | +2.0 | +2.5 | +2.8 |
Assumes the speedo was accurate on 255/40R19. Odometer records 97.5 km for every 100 km actually driven.
- The 245/40R20 is 17.4 mm (2.53 %) larger in diameter, so your speedo will read low and your odometer will under-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 19" to 20", so new wheels are required.
Is 245/40R20 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 245/40R20 is 17.4 mm larger than 255/40R19; 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 245/40R20 instead of 255/40R19?
Overall diameter changes by +17.4 mm (+2.53 %). 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 245/40R20?
Your speedo will read low by about 2.53 %. At an indicated 100 km/h you would actually be travelling 102.5 km/h, and at 110 km/h about 112.8 km/h.
What is the rolling circumference of 245/40R20?
Static circumference is 2212 mm; the ETRTO rolling circumference under load is about 2147 mm, which is 466 revolutions per kilometre.
245/40R20 in inches
245/40R20 is approximately 27.7×9.6R20: 27.7" tall, 9.6" wide, on a 20" rim, with a 3.9" sidewall.
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