235/55R19 vs 255/60R18
255/60R18 has an overall diameter of 763.2 mm (30.05") compared with 741.1 mm (29.18") for 235/55R19, a difference of +22.1 mm (+2.98 %). The sidewall changes by +23.8 mm, section width by +20 mm and ride height by +11.1 mm. Using the ETRTO rolling circumference, 235/55R19 turns 442 times per kilometre and 255/60R18 turns 430 times. With 255/60R18 fitted to a car calibrated for 235/55R19, an indicated 100 km/h is a true 103 km/h, and the odometer records 97.1 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 235/55R19 | 255/60R18 | Difference |
|---|---|---|---|
| Section width | 235 mm | 255 mm | +20 mm |
| Sidewall height | 129.3 mm | 153.0 mm | +23.8 mm |
| Rim diameter | 19" | 18" | -1.0" |
| Overall diameter | 741.1 mm (29.18") | 763.2 mm (30.05") | +22.1 mm (+2.98 %) |
| Static circumference | 2328 mm | 2398 mm | +69 mm |
| Rolling circumference (ETRTO) | 2260 mm | 2328 mm | +67 mm |
| Revs per km | 442 | 430 | -13 |
| Ride height | +11.1 mm | ||
| Inch equivalent | 29.2×9.3R19 | 30x10R18 | |
| Approved rim widths (approx.) | 6.5″–8.5″ | 7″–8.5″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 41.2 | 51.5 | 61.8 | 82.4 | 103.0 | 113.3 |
| Error (km/h) | +1.2 | +1.5 | +1.8 | +2.4 | +3.0 | +3.3 |
Assumes the speedo was accurate on 235/55R19. Odometer records 97.1 km for every 100 km actually driven.
- The 255/60R18 is 22.1 mm (2.98 %) 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 255/60R18 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 255/60R18 is 22.1 mm larger than 235/55R19; 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 103.0 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 3.0 % 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 255/60R18 instead of 235/55R19?
Overall diameter changes by +22.1 mm (+2.98 %). 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 255/60R18?
Your speedo will read low by about 2.98 %. At an indicated 100 km/h you would actually be travelling 103 km/h, and at 110 km/h about 113.3 km/h.
What is the rolling circumference of 255/60R18?
Static circumference is 2398 mm; the ETRTO rolling circumference under load is about 2328 mm, which is 430 revolutions per kilometre.
255/60R18 in inches
255/60R18 is approximately 30x10R18: 30.0" tall, 10.0" wide, on a 18" rim, with a 6.0" sidewall.
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