Comparison
255/40 R18 vs 255/45 R18
255/45 R18 is a taller-sidewall alternative to 255/40 R18 on the same 18-inch rim. This setup 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. Extra sidewall absorbs impacts more readily — a sensible bias for daily commuting and broken pavement. Diameter delta falls in the cautious 3–5% range, where speedometer recalibration and a careful clearance check are worth doing.
TakeA comfort-leaning direction that's easier on wheels and suspension over rough surfaces.
Quick math: 255/45 R18 is 25.5 mm taller than 255/40 R18, shifting the speedometer by +3.86%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
+25.5 mm
3.86%
Speedometer at 100
103.9 km/h
+3.86% error
Ground clearance
+12.8 mm
ride height delta
Sidewall change
+12.8 mm
revs/km: 463.5
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/compare/255-40-r18-vs-255-45-r18Detailed comparison
| Metric | 255/40 R18 | 255/45 R18 | Difference |
|---|---|---|---|
| Overall diameter | 661.2 mm | 686.7 mm | +25.5 mm (+3.86%) |
| Sidewall height | 102.0 mm | 114.8 mm | +12.8 mm |
| Circumference | 2.077 m | 2.157 m | +80.1 mm |
| Revs / km | 481.4 | 463.5 | -17.9 |
| Ground clearance | reference | +12.8 mm | +12.8 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 103.9 km/h | +3.86 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 103.9 km/h after switching to 255/45 R18 — a +3.86% 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 +12.8 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 R18
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255/45 R18
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