225/40R18 vs 205/60R16
205/60R16 has an overall diameter of 652.4 mm (25.69") compared with 637.2 mm (25.09") for 225/40R18, a difference of +15.2 mm (+2.39 %). The sidewall changes by +33.0 mm, section width by -20 mm and ride height by +7.6 mm. Using the ETRTO rolling circumference, 225/40R18 turns 515 times per kilometre and 205/60R16 turns 503 times. With 205/60R16 fitted to a car calibrated for 225/40R18, an indicated 100 km/h is a true 102.4 km/h, and the odometer records 97.7 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 225/40R18 | 205/60R16 | Difference |
|---|---|---|---|
| Section width | 225 mm | 205 mm | -20 mm |
| Sidewall height | 90.0 mm | 123.0 mm | +33.0 mm |
| Rim diameter | 18" | 16" | -2.0" |
| Overall diameter | 637.2 mm (25.09") | 652.4 mm (25.69") | +15.2 mm (+2.39 %) |
| Static circumference | 2002 mm | 2050 mm | +48 mm |
| Rolling circumference (ETRTO) | 1943 mm | 1990 mm | +46 mm |
| Revs per km | 515 | 503 | -12 |
| Ride height | +7.6 mm | ||
| Inch equivalent | 25.1×8.9R18 | 25.7×8.1R16 | |
| Approved rim widths (approx.) | 7.5″–9″ | 5.5″–7″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 41.0 | 51.2 | 61.4 | 81.9 | 102.4 | 112.6 |
| Error (km/h) | +1.0 | +1.2 | +1.4 | +1.9 | +2.4 | +2.6 |
Assumes the speedo was accurate on 225/40R18. Odometer records 97.7 km for every 100 km actually driven.
- The 205/60R16 is 15.2 mm (2.39 %) larger in diameter, so your speedo will read low and your odometer will under-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 18" to 16", so new wheels are required.
Is 205/60R16 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 205/60R16 is 15.2 mm larger than 225/40R18; the limit without certification for a passenger car (MA/MB) is +15 mm. Larger increases need an approved signatory / engineering certificate (modification code LS or state equivalent) and may not be permitted at all. | Cert. |
| Speedometer (ADR 18/03) A larger tyre makes the speedo read lower: at an indicated 100 km/h you would be doing 102.4 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 2.4 % increase is usually absorbed. Verify with a GPS speed app after fitting. | Verify |
| 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 205/60R16 instead of 225/40R18?
Overall diameter changes by +15.2 mm (+2.39 %). 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 205/60R16?
Your speedo will read low by about 2.39 %. At an indicated 100 km/h you would actually be travelling 102.4 km/h, and at 110 km/h about 112.6 km/h.
What is the rolling circumference of 205/60R16?
Static circumference is 2050 mm; the ETRTO rolling circumference under load is about 1990 mm, which is 503 revolutions per kilometre.
205/60R16 in inches
205/60R16 is approximately 25.7×8.1R16: 25.7" tall, 8.1" wide, on a 16" rim, with a 4.8" sidewall.
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