225/65R17 vs 235/60R18
235/60R18 has an overall diameter of 739.2 mm (29.10") compared with 724.3 mm (28.52") for 225/65R17, a difference of +14.9 mm (+2.06 %). The sidewall changes by -5.2 mm, section width by +10 mm and ride height by +7.5 mm. Using the ETRTO rolling circumference, 225/65R17 turns 453 times per kilometre and 235/60R18 turns 444 times. With 235/60R18 fitted to a car calibrated for 225/65R17, an indicated 100 km/h is a true 102.1 km/h, and the odometer records 98 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 225/65R17 | 235/60R18 | Difference |
|---|---|---|---|
| Section width | 225 mm | 235 mm | +10 mm |
| Sidewall height | 146.3 mm | 141.0 mm | -5.2 mm |
| Rim diameter | 17" | 18" | +1.0" |
| Overall diameter | 724.3 mm (28.52") | 739.2 mm (29.10") | +14.9 mm (+2.06 %) |
| Static circumference | 2275 mm | 2322 mm | +47 mm |
| Rolling circumference (ETRTO) | 2209 mm | 2255 mm | +45 mm |
| Revs per km | 453 | 444 | -9 |
| Ride height | +7.5 mm | ||
| Inch equivalent | 28.5×8.9R17 | 29.1×9.3R18 | |
| Approved rim widths (approx.) | 6″–7.5″ | 6.5″–8″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 40.8 | 51.0 | 61.2 | 81.6 | 102.1 | 112.3 |
| Error (km/h) | +0.8 | +1.0 | +1.2 | +1.6 | +2.1 | +2.3 |
Assumes the speedo was accurate on 225/65R17. Odometer records 98.0 km for every 100 km actually driven.
- The 235/60R18 is 14.9 mm (2.06 %) larger in diameter, so your speedo will read low and your odometer will under-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.
- Rim diameter changes from 17" to 18", so new wheels are required.
Is 235/60R18 legal in Australia? Within limits, verify speedo
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of +14.9 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 larger tyre makes the speedo read lower: at an indicated 100 km/h you would be doing 102.1 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 2.1 % 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 235/60R18 instead of 225/65R17?
Overall diameter changes by +14.9 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 235/60R18?
Your speedo will read low by about 2.06 %. At an indicated 100 km/h you would actually be travelling 102.1 km/h, and at 110 km/h about 112.3 km/h.
What is the rolling circumference of 235/60R18?
Static circumference is 2322 mm; the ETRTO rolling circumference under load is about 2255 mm, which is 444 revolutions per kilometre.
235/60R18 in inches
235/60R18 is approximately 29.1×9.3R18: 29.1" tall, 9.3" wide, on a 18" rim, with a 5.6" sidewall.
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