Comparison
235/55 R18 vs 255/45 R20
Switching from 235/55 R18 to 255/45 R20 is a plus-2 upgrade that wraps a shorter sidewall around a larger 20-inch wheel. This tire combination noticeably changes overall diameter compared to OEM. Less sidewall flex usually translates to crisper turn-in and a slightly stiffer ride over rough pavement.
Indicated speed will drift far enough that recalibration is worth considering. Diameter delta falls in the cautious 3–5% range, where speedometer recalibration and a careful clearance check are worth doing.
TakeCommon upgrade for sportier handling and a tighter wheel-gap look on the same vehicle.
Quick math: 255/45 R20 is 21.8 mm taller than 235/55 R18, shifting the speedometer by +3.05%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
+21.8 mm
3.05%
Speedometer at 100
103.0 km/h
+3.05% error
Ground clearance
+10.9 mm
ride height delta
Sidewall change
-14.5 mm
revs/km: 431.6
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/compare/235-55-r18-vs-255-45-r20Detailed comparison
| Metric | 235/55 R18 | 255/45 R20 | Difference |
|---|---|---|---|
| Overall diameter | 715.7 mm | 737.5 mm | +21.8 mm (+3.05%) |
| Sidewall height | 129.3 mm | 114.8 mm | -14.5 mm |
| Circumference | 2.248 m | 2.317 m | +68.5 mm |
| Revs / km | 444.8 | 431.6 | -13.1 |
| Ground clearance | reference | +10.9 mm | +10.9 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 103.0 km/h | +3.05 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 103.0 km/h after switching to 255/45 R20 — a +3.05% 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 +10.9 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.
Back to
235/55 R18
Back to
255/45 R20
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