235/50R18 vs 245/45R18
245/45R18 has an overall diameter of 677.7 mm (26.68") compared with 692.2 mm (27.25") for 235/50R18, a difference of -14.5 mm (-2.09 %). The sidewall changes by -7.2 mm, section width by +10 mm and ride height by -7.2 mm. Using the ETRTO rolling circumference, 235/50R18 turns 474 times per kilometre and 245/45R18 turns 484 times. With 245/45R18 fitted to a car calibrated for 235/50R18, an indicated 100 km/h is a true 97.9 km/h, and the odometer records 102.1 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 235/50R18 | 245/45R18 | Difference |
|---|---|---|---|
| Section width | 235 mm | 245 mm | +10 mm |
| Sidewall height | 117.5 mm | 110.3 mm | -7.2 mm |
| Rim diameter | 18" | 18" | 0.0" |
| Overall diameter | 692.2 mm (27.25") | 677.7 mm (26.68") | -14.5 mm (-2.09 %) |
| Static circumference | 2175 mm | 2129 mm | -46 mm |
| Rolling circumference (ETRTO) | 2111 mm | 2067 mm | -44 mm |
| Revs per km | 474 | 484 | +10 |
| Ride height | -7.2 mm | ||
| Inch equivalent | 27.3×9.3R18 | 26.7×9.6R18 | |
| Approved rim widths (approx.) | 6.5″–8.5″ | 7.5″–9″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.2 | 49.0 | 58.7 | 78.3 | 97.9 | 107.7 |
| Error (km/h) | -0.8 | -1.0 | -1.3 | -1.7 | -2.1 | -2.3 |
Assumes the speedo was accurate on 235/50R18. Odometer records 102.1 km for every 100 km actually driven.
- The 245/45R18 is 14.5 mm (2.09 %) 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.
Is 245/45R18 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -14.5 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 245/45R18 instead of 235/50R18?
Overall diameter changes by -14.5 mm (-2.09 %). 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 245/45R18?
Your speedo will read high by about 2.09 %. 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 245/45R18?
Static circumference is 2129 mm; the ETRTO rolling circumference under load is about 2067 mm, which is 484 revolutions per kilometre.
245/45R18 in inches
245/45R18 is approximately 26.7×9.6R18: 26.7" tall, 9.6" wide, on a 18" rim, with a 4.3" 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.