Comparison
235/55 R18 vs 255/35 R20
Switching from 235/55 R18 to 255/35 R20 is a plus-2 upgrade that wraps a shorter sidewall around a larger 20-inch wheel. This alternative fitment 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. The 3–5% diameter gap puts this in caution territory: doable on many cars, but verify clearance and consider recalibration.
TakeA solid pick for drivers chasing a more aggressive stance without abandoning OEM rolling diameter.
Quick math: 255/35 R20 is 29.2 mm shorter than 235/55 R18, shifting the speedometer by -4.08%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
-29.2 mm
-4.08%
Speedometer at 100
95.9 km/h
-4.08% error
Ground clearance
-14.6 mm
ride height delta
Sidewall change
-40.0 mm
revs/km: 463.7
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/compare/235-55-r18-vs-255-35-r20Detailed comparison
| Metric | 235/55 R18 | 255/35 R20 | Difference |
|---|---|---|---|
| Overall diameter | 715.7 mm | 686.5 mm | -29.2 mm (-4.08%) |
| Sidewall height | 129.3 mm | 89.3 mm | -40.0 mm |
| Circumference | 2.248 m | 2.157 m | -91.7 mm |
| Revs / km | 444.8 | 463.7 | +18.9 |
| Ground clearance | reference | -14.6 mm | -14.6 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 95.9 km/h | -4.08 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 95.9 km/h after switching to 255/35 R20 — a -4.08% 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 -14.6 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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235/55 R18
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255/35 R20
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