Comparison
255/40 R17 vs 255/40 R18
255/40 R18 is a plus-1 alternative to 255/40 R17 — the bigger wheel shows through a thinner sidewall. This alternative fitment moves rolling diameter well outside the usual OEM tolerance. The larger wheel shows more of the brake hardware and tightens up the wheel-gap look.
The speedometer error is noticeable and may warrant a recalibration if you rely on indicated speed. The 3–5% diameter gap puts this in caution territory: doable on many cars, but verify clearance and consider recalibration.
TakeCommon upgrade for sportier handling and a tighter wheel-gap look on the same vehicle.
Quick math: 255/40 R18 is 25.4 mm taller than 255/40 R17, shifting the speedometer by +3.99%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
+25.4 mm
3.99%
Speedometer at 100
104.0 km/h
+3.99% error
Ground clearance
+12.7 mm
ride height delta
Sidewall change
0.0 mm
revs/km: 481.4
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/compare/255-40-r17-vs-255-40-r18Detailed comparison
| Metric | 255/40 R17 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 635.8 mm | 661.2 mm | +25.4 mm (+3.99%) |
| Sidewall height | 102.0 mm | 102.0 mm | 0.0 mm |
| Circumference | 1.997 m | 2.077 m | +79.8 mm |
| Revs / km | 500.6 | 481.4 | -19.2 |
| Ground clearance | reference | +12.7 mm | +12.7 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 104.0 km/h | +3.99 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 104.0 km/h after switching to 255/40 R18 — a +3.99% 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 +12.7 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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255/40 R17
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255/40 R18
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