Comparison
255/35 R20 vs 255/40 R18
Switching from 255/35 R20 to 255/40 R18 steps down to a 18-inch wheel — a familiar move for winter and dedicated all-terrain sets. This setup moves rolling diameter well outside the usual OEM tolerance. Indicated speed will drift far enough that recalibration is worth considering. The taller sidewall adds cushioning over potholes and rougher roads, with a softer overall ride. The 3–5% diameter gap puts this in caution territory: doable on many cars, but verify clearance and consider recalibration.
TakeTypical choice for a dedicated winter or off-road setup where extra sidewall pays off.
Quick math: 255/40 R18 is 25.3 mm shorter than 255/35 R20, shifting the speedometer by -3.69%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
-25.3 mm
-3.69%
Speedometer at 100
96.3 km/h
-3.69% error
Ground clearance
-12.6 mm
ride height delta
Sidewall change
+12.8 mm
revs/km: 481.4
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/compare/255-35-r20-vs-255-40-r18Detailed comparison
| Metric | 255/35 R20 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 686.5 mm | 661.2 mm | -25.3 mm (-3.69%) |
| Sidewall height | 89.3 mm | 102.0 mm | +12.8 mm |
| Circumference | 2.157 m | 2.077 m | -79.5 mm |
| Revs / km | 463.7 | 481.4 | +17.7 |
| Ground clearance | reference | -12.6 mm | -12.6 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 96.3 km/h | -3.69 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 96.3 km/h after switching to 255/40 R18 — a -3.69% 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 -12.6 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.
Back to
255/35 R20
Back to
255/40 R18
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