225/50R16 vs 235/40R18
235/40R18 has an overall diameter of 645.2 mm (25.40") compared with 631.4 mm (24.86") for 225/50R16, a difference of +13.8 mm (+2.19 %). The sidewall changes by -18.5 mm, section width by +10 mm and ride height by +6.9 mm. Using the ETRTO rolling circumference, 225/50R16 turns 519 times per kilometre and 235/40R18 turns 508 times. With 235/40R18 fitted to a car calibrated for 225/50R16, an indicated 100 km/h is a true 102.2 km/h, and the odometer records 97.9 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 225/50R16 | 235/40R18 | Difference |
|---|---|---|---|
| Section width | 225 mm | 235 mm | +10 mm |
| Sidewall height | 112.5 mm | 94.0 mm | -18.5 mm |
| Rim diameter | 16" | 18" | +2.0" |
| Overall diameter | 631.4 mm (24.86") | 645.2 mm (25.40") | +13.8 mm (+2.19 %) |
| Static circumference | 1984 mm | 2027 mm | +43 mm |
| Rolling circumference (ETRTO) | 1926 mm | 1968 mm | +42 mm |
| Revs per km | 519 | 508 | -11 |
| Ride height | +6.9 mm | ||
| Inch equivalent | 24.9×8.9R16 | 25.4×9.3R18 | |
| Approved rim widths (approx.) | 6″–8″ | 8″–9.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 40.9 | 51.1 | 61.3 | 81.7 | 102.2 | 112.4 |
| Error (km/h) | +0.9 | +1.1 | +1.3 | +1.7 | +2.2 | +2.4 |
Assumes the speedo was accurate on 225/50R16. Odometer records 97.9 km for every 100 km actually driven.
- The 235/40R18 is 13.8 mm (2.19 %) 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 16" to 18", so new wheels are required.
Is 235/40R18 legal in Australia? Within limits, verify speedo
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of +13.8 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 102.2 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 2.2 % 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 235/40R18 instead of 225/50R16?
Overall diameter changes by +13.8 mm (+2.19 %). 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 235/40R18?
Your speedo will read low by about 2.19 %. At an indicated 100 km/h you would actually be travelling 102.2 km/h, and at 110 km/h about 112.4 km/h.
What is the rolling circumference of 235/40R18?
Static circumference is 2027 mm; the ETRTO rolling circumference under load is about 1968 mm, which is 508 revolutions per kilometre.
235/40R18 in inches
235/40R18 is approximately 25.4×9.3R18: 25.4" tall, 9.3" wide, on a 18" rim, with a 3.7" sidewall.
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