205/60R16 vs 225/40R18
225/40R18 has an overall diameter of 637.2 mm (25.09") compared with 652.4 mm (25.69") for 205/60R16, a difference of -15.2 mm (-2.33 %). The sidewall changes by -33.0 mm, section width by +20 mm and ride height by -7.6 mm. Using the ETRTO rolling circumference, 205/60R16 turns 503 times per kilometre and 225/40R18 turns 515 times. With 225/40R18 fitted to a car calibrated for 205/60R16, an indicated 100 km/h is a true 97.7 km/h, and the odometer records 102.4 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 205/60R16 | 225/40R18 | Difference |
|---|---|---|---|
| Section width | 205 mm | 225 mm | +20 mm |
| Sidewall height | 123.0 mm | 90.0 mm | -33.0 mm |
| Rim diameter | 16" | 18" | +2.0" |
| Overall diameter | 652.4 mm (25.69") | 637.2 mm (25.09") | -15.2 mm (-2.33 %) |
| Static circumference | 2050 mm | 2002 mm | -48 mm |
| Rolling circumference (ETRTO) | 1990 mm | 1943 mm | -46 mm |
| Revs per km | 503 | 515 | +12 |
| Ride height | -7.6 mm | ||
| Inch equivalent | 25.7×8.1R16 | 25.1×8.9R18 | |
| Approved rim widths (approx.) | 5.5″–7″ | 7.5″–9″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.1 | 48.8 | 58.6 | 78.1 | 97.7 | 107.4 |
| Error (km/h) | -0.9 | -1.2 | -1.4 | -1.9 | -2.3 | -2.6 |
Assumes the speedo was accurate on 205/60R16. Odometer records 102.4 km for every 100 km actually driven.
- The 225/40R18 is 15.2 mm (2.33 %) smaller in diameter, so your speedo will read high and your odometer will over-record distance.
- Section width changes by +20 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 16" to 18", so new wheels are required.
Is 225/40R18 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -15.2 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 97.7 km/h (over-read 2.3 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 225/40R18 instead of 205/60R16?
Overall diameter changes by -15.2 mm (-2.33 %). 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 225/40R18?
Your speedo will read high by about 2.33 %. At an indicated 100 km/h you would actually be travelling 97.7 km/h, and at 110 km/h about 107.4 km/h.
What is the rolling circumference of 225/40R18?
Static circumference is 2002 mm; the ETRTO rolling circumference under load is about 1943 mm, which is 515 revolutions per kilometre.
225/40R18 in inches
225/40R18 is approximately 25.1×8.9R18: 25.1" tall, 8.9" wide, on a 18" rim, with a 3.5" sidewall.
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