Comparison
235/50 R18 vs 255/40 R18
Switching from 235/50 R18 to 255/40 R18 drops the aspect ratio by 10 points on the same 18-inch wheel. This tire combination swings rolling diameter far enough to feel on the road. The speedometer error is noticeable and may warrant a recalibration if you rely on indicated speed. The shorter sidewall gives the tire a firmer, more responsive feel and sharpens steering input. Diameter delta falls in the cautious 3–5% range, where speedometer recalibration and a careful clearance check are worth doing.
TakeSuits drivers who value sharper steering and appearance over outright ride softness.
Quick math: 255/40 R18 is 31.0 mm shorter than 235/50 R18, shifting the speedometer by -4.48%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
-31.0 mm
-4.48%
Speedometer at 100
95.5 km/h
-4.48% error
Ground clearance
-15.5 mm
ride height delta
Sidewall change
-15.5 mm
revs/km: 481.4
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/compare/235-50-r18-vs-255-40-r18Detailed comparison
| Metric | 235/50 R18 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 692.2 mm | 661.2 mm | -31.0 mm (-4.48%) |
| Sidewall height | 117.5 mm | 102.0 mm | -15.5 mm |
| Circumference | 2.175 m | 2.077 m | -97.4 mm |
| Revs / km | 459.9 | 481.4 | +21.6 |
| Ground clearance | reference | -15.5 mm | -15.5 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 95.5 km/h | -4.48 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 95.5 km/h after switching to 255/40 R18 — a -4.48% 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 -15.5 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.
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235/50 R18
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255/40 R18
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