Comparison
235/55 R17 vs 255/40 R18
Switching from 235/55 R17 to 255/40 R18 is a plus-1 upgrade that wraps a shorter sidewall around a larger 18-inch wheel. This alternative fitment swings rolling diameter far enough to feel on the road.
Indicated speed will drift far enough that recalibration is worth considering. Less sidewall flex usually translates to crisper turn-in and a slightly stiffer ride over rough pavement. The wider section adds contact patch and lateral stability, while eating into fender and suspension clearance. Visually, the bigger wheel fills the arch and gives the car a more aggressive stance. Diameter delta falls in the cautious 3–5% range, where speedometer recalibration and a careful clearance check are worth doing.
TakeA solid pick for drivers chasing a more aggressive stance without abandoning OEM rolling diameter.
Quick math: 255/40 R18 is 29.1 mm shorter than 235/55 R17, shifting the speedometer by -4.22%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
-29.1 mm
-4.22%
Speedometer at 100
95.8 km/h
-4.22% error
Ground clearance
-14.5 mm
ride height delta
Sidewall change
-27.3 mm
revs/km: 481.4
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/compare/235-55-r17-vs-255-40-r18Detailed comparison
| Metric | 235/55 R17 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 690.3 mm | 661.2 mm | -29.1 mm (-4.22%) |
| Sidewall height | 129.3 mm | 102.0 mm | -27.3 mm |
| Circumference | 2.169 m | 2.077 m | -91.4 mm |
| Revs / km | 461.1 | 481.4 | +20.3 |
| Ground clearance | reference | -14.5 mm | -14.5 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 95.8 km/h | -4.22 km/h |
Verdict: warning
Between 3% and 5% — noticeable speedometer drift; recalibration may be advisable.
Speedometer impact
At a true 100 km/h, your dashboard will read 95.8 km/h after switching to 255/40 R18 — a -4.22% offset. Use the speedometer error calculator to recheck for other indicated speeds, and the speedometer error guide for the full background.
Ground clearance change
The new tire's half-diameter changes ride height by -14.5 mm. Small differences are absorbed by suspension travel, but anything beyond ±10 mm can affect headlight aim, fender clearance and bump-stop margin. See the plus-sizing guide before committing.
Back to
235/55 R17
Back to
255/40 R18
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