235/40R18 vs 205/55R16
205/55R16 has an overall diameter of 631.9 mm (24.88") compared with 645.2 mm (25.40") for 235/40R18, a difference of -13.3 mm (-2.06 %). The sidewall changes by +18.8 mm, section width by -30 mm and ride height by -6.7 mm. Using the ETRTO rolling circumference, 235/40R18 turns 508 times per kilometre and 205/55R16 turns 519 times. With 205/55R16 fitted to a car calibrated for 235/40R18, 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 | 235/40R18 | 205/55R16 | Difference |
|---|---|---|---|
| Section width | 235 mm | 205 mm | -30 mm |
| Sidewall height | 94.0 mm | 112.8 mm | +18.8 mm |
| Rim diameter | 18" | 16" | -2.0" |
| Overall diameter | 645.2 mm (25.40") | 631.9 mm (24.88") | -13.3 mm (-2.06 %) |
| Static circumference | 2027 mm | 1985 mm | -42 mm |
| Rolling circumference (ETRTO) | 1968 mm | 1927 mm | -41 mm |
| Revs per km | 508 | 519 | +11 |
| Ride height | -6.7 mm | ||
| Inch equivalent | 25.4×9.3R18 | 24.9×8.1R16 | |
| Approved rim widths (approx.) | 8″–9.5″ | 5.5″–7.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 39.2 | 49.0 | 58.8 | 78.4 | 97.9 | 107.7 |
| Error (km/h) | -0.8 | -1.0 | -1.2 | -1.6 | -2.1 | -2.3 |
Assumes the speedo was accurate on 235/40R18. Odometer records 102.1 km for every 100 km actually driven.
- The 205/55R16 is 13.3 mm (2.06 %) smaller in diameter, so your speedo will read high and your odometer will over-record distance.
- Section width changes by -30 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 16", so new wheels are required.
Is 205/55R16 legal in Australia? More info needed
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of -13.3 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 205/55R16 instead of 235/40R18?
Overall diameter changes by -13.3 mm (-2.06 %). 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 205/55R16?
Your speedo will read high by about 2.06 %. 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 205/55R16?
Static circumference is 1985 mm; the ETRTO rolling circumference under load is about 1927 mm, which is 519 revolutions per kilometre.
205/55R16 in inches
205/55R16 is approximately 24.9×8.1R16: 24.9" tall, 8.1" wide, on a 16" rim, with a 4.4" sidewall.
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