235/40R19 vs 235/45R18
235/45R18 has an overall diameter of 668.7 mm (26.33") compared with 670.6 mm (26.40") for 235/40R19, a difference of -1.9 mm (-0.28 %). The sidewall changes by +11.8 mm, section width by 0 mm and ride height by -0.9 mm. Using the ETRTO rolling circumference, 235/40R19 turns 489 times per kilometre and 235/45R18 turns 490 times. With 235/45R18 fitted to a car calibrated for 235/40R19, an indicated 100 km/h is a true 99.7 km/h, and the odometer records 100.3 km for every 100 km driven. This is within the commonly quoted 3 % tolerance.
Full comparison
| Measurement | 235/40R19 | 235/45R18 | Difference |
|---|---|---|---|
| Section width | 235 mm | 235 mm | 0 mm |
| Sidewall height | 94.0 mm | 105.8 mm | +11.8 mm |
| Rim diameter | 19" | 18" | -1.0" |
| Overall diameter | 670.6 mm (26.40") | 668.7 mm (26.33") | -1.9 mm (-0.28 %) |
| Static circumference | 2107 mm | 2101 mm | -6 mm |
| Rolling circumference (ETRTO) | 2045 mm | 2040 mm | -6 mm |
| Revs per km | 489 | 490 | +1 |
| Ride height | -0.9 mm | ||
| Inch equivalent | 26.4×9.3R19 | 26.3×9.3R18 | |
| Approved rim widths (approx.) | 8″–9.5″ | 7.5″–9″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.9 | 49.9 | 59.8 | 79.8 | 99.7 | 109.7 |
| Error (km/h) | -0.1 | -0.1 | -0.2 | -0.2 | -0.3 | -0.3 |
Assumes the speedo was accurate on 235/40R19. Odometer records 100.3 km for every 100 km actually driven.
- The 235/45R18 is 1.9 mm (0.28 %) smaller in diameter, so your speedo will read high and your odometer will over-record distance.
- Rim diameter changes from 19" to 18", so new wheels are required.
Is 235/45R18 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -1.9 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 99.7 km/h (over-read 0.3 km/h; ADR 18 allows up to 14.0 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 235/45R18 instead of 235/40R19?
Overall diameter changes by -1.9 mm (-0.28 %). That is within the usual 3 % rule of thumb, so it is generally an acceptable swap provided the load index and speed rating meet your tyre placard and the tyre suits your rim width. 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 235/45R18?
Your speedo will read high by about 0.28 %. At an indicated 100 km/h you would actually be travelling 99.7 km/h, and at 110 km/h about 109.7 km/h.
What is the rolling circumference of 235/45R18?
Static circumference is 2101 mm; the ETRTO rolling circumference under load is about 2040 mm, which is 490 revolutions per kilometre.
235/45R18 in inches
235/45R18 is approximately 26.3×9.3R18: 26.3" tall, 9.3" wide, on a 18" rim, with a 4.2" sidewall.
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