Comparison
235/40 R18 vs 255/40 R17
Switching from 235/40 R18 to 255/40 R17 steps down to a 17-inch wheel — a familiar move for winter and dedicated all-terrain sets. This swap barely shifts the rolling circumference. Speedometer drift stays small enough that most drivers won't notice it day to day. More tread on the ground tends to improve dry grip and stance, with a small fuel-economy and clearance tradeoff. Diameter change stays inside the conservative ±3% safety window — an OEM-safe fitment on most vehicles.
TakeTypical choice for a dedicated winter or off-road setup where extra sidewall pays off.
Quick math: 255/40 R17 is 9.4 mm shorter than 235/40 R18, shifting the speedometer by -1.46%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
-9.4 mm
-1.46%
Speedometer at 100
98.5 km/h
-1.46% error
Ground clearance
-4.7 mm
ride height delta
Sidewall change
+8.0 mm
revs/km: 500.6
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/compare/235-40-r18-vs-255-40-r17Detailed comparison
| Metric | 235/40 R18 | 255/40 R17 | Difference |
|---|---|---|---|
| Overall diameter | 645.2 mm | 635.8 mm | -9.4 mm (-1.46%) |
| Sidewall height | 94.0 mm | 102.0 mm | +8.0 mm |
| Circumference | 2.027 m | 1.997 m | -29.5 mm |
| Revs / km | 493.4 | 500.6 | +7.3 |
| Ground clearance | reference | -4.7 mm | -4.7 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 98.5 km/h | -1.46 km/h |
Verdict: excellent
Within ±3% — speedometer, ABS and traction control should behave normally.
Speedometer impact
At a true 100 km/h, your dashboard will read 98.5 km/h after switching to 255/40 R17 — a -1.46% 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 -4.7 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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235/40 R18
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255/40 R17
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