255/40R18 vs 225/55R17
225/55R17 has an overall diameter of 679.3 mm (26.74") compared with 661.2 mm (26.03") for 255/40R18, a difference of +18.1 mm (+2.74 %). The sidewall changes by +21.8 mm, section width by -30 mm and ride height by +9.0 mm. Using the ETRTO rolling circumference, 255/40R18 turns 496 times per kilometre and 225/55R17 turns 483 times. With 225/55R17 fitted to a car calibrated for 255/40R18, an indicated 100 km/h is a true 102.7 km/h, and the odometer records 97.3 km for every 100 km driven. This is close to the commonly quoted 3 % tolerance, so check clearance and local rules.
Full comparison
| Measurement | 255/40R18 | 225/55R17 | Difference |
|---|---|---|---|
| Section width | 255 mm | 225 mm | -30 mm |
| Sidewall height | 102.0 mm | 123.8 mm | +21.8 mm |
| Rim diameter | 18" | 17" | -1.0" |
| Overall diameter | 661.2 mm (26.03") | 679.3 mm (26.74") | +18.1 mm (+2.74 %) |
| Static circumference | 2077 mm | 2134 mm | +57 mm |
| Rolling circumference (ETRTO) | 2017 mm | 2072 mm | +55 mm |
| Revs per km | 496 | 483 | -13 |
| Ride height | +9.0 mm | ||
| Inch equivalent | 26x10R18 | 26.7×8.9R17 | |
| Approved rim widths (approx.) | 8.5″–10″ | 6″–8″ |
Speedometer error
| Speedo reads | 40 | 50 | 60 | 80 | 100 | 110 |
|---|---|---|---|---|---|---|
| Actual speed (km/h) | 41.1 | 51.4 | 61.6 | 82.2 | 102.7 | 113.0 |
| Error (km/h) | +1.1 | +1.4 | +1.6 | +2.2 | +2.7 | +3.0 |
Assumes the speedo was accurate on 255/40R18. Odometer records 97.3 km for every 100 km actually driven.
- The 225/55R17 is 18.1 mm (2.74 %) larger in diameter, so your speedo will read low and your odometer will under-record distance.
- Section width changes by -30 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 17", so new wheels are required.
Is 225/55R17 legal in Australia? Needs certification
| Overall diameter (VSB 14 LS 4.2.4) 225/55R17 is 18.1 mm larger than 255/40R18; 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 102.7 km/h. ADR 18 requires the speedo never to read below true speed, but factory speedos typically over-read by 2-6 %, so a 2.7 % 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 225/55R17 instead of 255/40R18?
Overall diameter changes by +18.1 mm (+2.74 %). 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 225/55R17?
Your speedo will read low by about 2.74 %. At an indicated 100 km/h you would actually be travelling 102.7 km/h, and at 110 km/h about 113 km/h.
What is the rolling circumference of 225/55R17?
Static circumference is 2134 mm; the ETRTO rolling circumference under load is about 2072 mm, which is 483 revolutions per kilometre.
225/55R17 in inches
225/55R17 is approximately 26.7×8.9R17: 26.7" tall, 8.9" wide, on a 17" rim, with a 4.9" sidewall.
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