235/35R19 vs 255/40R18
255/40R18 has an overall diameter of 661.2 mm (26.03") compared with 647.1 mm (25.48") for 235/35R19, a difference of +14.1 mm (+2.18 %). The sidewall changes by +19.8 mm, section width by +20 mm and ride height by +7.1 mm. Using the ETRTO rolling circumference, 235/35R19 turns 507 times per kilometre and 255/40R18 turns 496 times. With 255/40R18 fitted to a car calibrated for 235/35R19, 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 | 235/35R19 | 255/40R18 | Difference |
|---|---|---|---|
| Section width | 235 mm | 255 mm | +20 mm |
| Sidewall height | 82.3 mm | 102.0 mm | +19.8 mm |
| Rim diameter | 19" | 18" | -1.0" |
| Overall diameter | 647.1 mm (25.48") | 661.2 mm (26.03") | +14.1 mm (+2.18 %) |
| Static circumference | 2033 mm | 2077 mm | +44 mm |
| Rolling circumference (ETRTO) | 1974 mm | 2017 mm | +43 mm |
| Revs per km | 507 | 496 | -11 |
| Ride height | +7.1 mm | ||
| Inch equivalent | 25.5×9.3R19 | 26x10R18 | |
| Approved rim widths (approx.) | 8″–9.5″ | 8.5″–10″ |
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 235/35R19. Odometer records 97.9 km for every 100 km actually driven.
- The 255/40R18 is 14.1 mm (2.18 %) 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 19" to 18", so new wheels are required.
Is 255/40R18 legal in Australia? Within limits, verify speedo
| Overall diameter (VSB 14 LS 4.2.4) Diameter change of +14.1 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 255/40R18 instead of 235/35R19?
Overall diameter changes by +14.1 mm (+2.18 %). 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 255/40R18?
Your speedo will read low by about 2.18 %. 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 255/40R18?
Static circumference is 2077 mm; the ETRTO rolling circumference under load is about 2017 mm, which is 496 revolutions per kilometre.
255/40R18 in inches
255/40R18 is approximately 26x10R18: 26.0" tall, 10.0" wide, on a 18" rim, with a 4.0" sidewall.
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