Comparison
205/50 R17 vs 255/40 R18
Going from 205/50 R17 to 255/40 R18 steps up to a 18-inch rim while trimming sidewall to stay near OEM rolling diameter. This wheel and tire pairing swings rolling diameter far enough to feel on the road. Less sidewall flex usually translates to crisper turn-in and a slightly stiffer ride over rough pavement.
The speedometer error is noticeable and may warrant a recalibration if you rely on indicated speed. 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 24.4 mm taller than 205/50 R17, shifting the speedometer by +3.83%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
+24.4 mm
3.83%
Speedometer at 100
103.8 km/h
+3.83% error
Ground clearance
+12.2 mm
ride height delta
Sidewall change
-0.5 mm
revs/km: 481.4
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/compare/205-50-r17-vs-255-40-r18Detailed comparison
| Metric | 205/50 R17 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 636.8 mm | 661.2 mm | +24.4 mm (+3.83%) |
| Sidewall height | 102.5 mm | 102.0 mm | -0.5 mm |
| Circumference | 2.001 m | 2.077 m | +76.7 mm |
| Revs / km | 499.9 | 481.4 | -18.4 |
| Ground clearance | reference | +12.2 mm | +12.2 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 103.8 km/h | +3.83 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 103.8 km/h after switching to 255/40 R18 — a +3.83% 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 +12.2 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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205/50 R17
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255/40 R18
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