Comparison
235/45 R17 vs 255/40 R18
255/40 R18 is a plus-1 alternative to 235/45 R17 — the bigger wheel shows through a thinner sidewall. This wheel and tire pairing moves rolling diameter a touch off the original spec. Less sidewall flex usually translates to crisper turn-in and a slightly stiffer ride over rough pavement.
The speedometer offset is small but measurable; worth keeping in mind if you watch the dash closely. Overall the swap sits inside the safe ±3% diameter window, so ABS, traction control and gearing behave normally.
TakeA solid pick for drivers chasing a more aggressive stance without abandoning OEM rolling diameter.
Quick math: 255/40 R18 is 17.9 mm taller than 235/45 R17, shifting the speedometer by +2.78%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
+17.9 mm
2.78%
Speedometer at 100
102.8 km/h
+2.78% error
Ground clearance
+9.0 mm
ride height delta
Sidewall change
-3.8 mm
revs/km: 481.4
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/compare/235-45-r17-vs-255-40-r18Detailed comparison
| Metric | 235/45 R17 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 643.3 mm | 661.2 mm | +17.9 mm (+2.78%) |
| Sidewall height | 105.8 mm | 102.0 mm | -3.8 mm |
| Circumference | 2.021 m | 2.077 m | +56.2 mm |
| Revs / km | 494.8 | 481.4 | -13.4 |
| Ground clearance | reference | +9.0 mm | +9.0 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 102.8 km/h | +2.78 km/h |
Verdict: excellent
Within ±3% — speedometer, ABS and traction control should behave normally.
Speedometer impact
At a true 100 km/h, your dashboard will read 102.8 km/h after switching to 255/40 R18 — a +2.78% 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 +9.0 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.
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235/45 R17
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255/40 R18
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