Comparison
255/35 R18 vs 255/40 R18
Going from 255/35 R18 to 255/40 R18 raises the aspect ratio by 5 points on the same 18-inch wheel. This wheel and tire pairing moves rolling diameter well outside the usual OEM tolerance. The speedometer error is noticeable and may warrant a recalibration if you rely on indicated speed. More sidewall typically improves comfort and curb protection, especially on city streets. 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: 255/40 R18 is 25.5 mm taller than 255/35 R18, shifting the speedometer by +4.01%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
+25.5 mm
4.01%
Speedometer at 100
104.0 km/h
+4.01% error
Ground clearance
+12.8 mm
ride height delta
Sidewall change
+12.8 mm
revs/km: 481.4
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/compare/255-35-r18-vs-255-40-r18Detailed comparison
| Metric | 255/35 R18 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 635.7 mm | 661.2 mm | +25.5 mm (+4.01%) |
| Sidewall height | 89.3 mm | 102.0 mm | +12.8 mm |
| Circumference | 1.997 m | 2.077 m | +80.1 mm |
| Revs / km | 500.7 | 481.4 | -19.3 |
| Ground clearance | reference | +12.8 mm | +12.8 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 104.0 km/h | +4.01 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 104.0 km/h after switching to 255/40 R18 — a +4.01% 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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255/35 R18
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255/40 R18
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