235/40R18 vs 245/40R17
245/40R17 has an overall diameter of 627.8 mm (24.72") compared with 645.2 mm (25.40") for 235/40R18, a difference of -17.4 mm (-2.70 %). The sidewall changes by +4.0 mm, section width by +10 mm and ride height by -8.7 mm. Using the ETRTO rolling circumference, 235/40R18 turns 508 times per kilometre and 245/40R17 turns 522 times. With 245/40R17 fitted to a car calibrated for 235/40R18, an indicated 100 km/h is a true 97.3 km/h, and the odometer records 102.8 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 235/40R18 | 245/40R17 | Difference |
|---|---|---|---|
| Section width | 235 mm | 245 mm | +10 mm |
| Sidewall height | 94.0 mm | 98.0 mm | +4.0 mm |
| Rim diameter | 18" | 17" | -1.0" |
| Overall diameter | 645.2 mm (25.40") | 627.8 mm (24.72") | -17.4 mm (-2.70 %) |
| Static circumference | 2027 mm | 1972 mm | -55 mm |
| Rolling circumference (ETRTO) | 1968 mm | 1915 mm | -53 mm |
| Revs per km | 508 | 522 | +14 |
| Ride height | -8.7 mm | ||
| Inch equivalent | 25.4×9.3R18 | 24.7×9.6R17 | |
| Approved rim widths (approx.) | 8″–9.5″ | 8″–9.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 38.9 | 48.7 | 58.4 | 77.8 | 97.3 | 107.0 |
| Error (km/h) | -1.1 | -1.3 | -1.6 | -2.2 | -2.7 | -3.0 |
Assumes the speedo was accurate on 235/40R18. Odometer records 102.8 km for every 100 km actually driven.
- The 245/40R17 is 17.4 mm (2.7 %) 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 18" to 17", so new wheels are required.
Is 245/40R17 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -17.4 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 97.3 km/h (over-read 2.7 km/h; ADR 18 allows up to 13.7 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 245/40R17 instead of 235/40R18?
Overall diameter changes by -17.4 mm (-2.70 %). 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 245/40R17?
Your speedo will read high by about 2.70 %. At an indicated 100 km/h you would actually be travelling 97.3 km/h, and at 110 km/h about 107 km/h.
What is the rolling circumference of 245/40R17?
Static circumference is 1972 mm; the ETRTO rolling circumference under load is about 1915 mm, which is 522 revolutions per kilometre.
245/40R17 in inches
245/40R17 is approximately 24.7×9.6R17: 24.7" tall, 9.6" wide, on a 17" rim, with a 3.9" sidewall.
Buying tyres for your business?
Onyx Tyres supplies workshops, retailers and fleets across Australia at wholesale prices. Open a trade account to see stock and pricing and order online.