Comparison
235/45 R18 vs 255/40 R17
Switching from 235/45 R18 to 255/40 R17 steps down to a 17-inch wheel — a familiar move for winter and dedicated all-terrain sets. This setup noticeably changes overall diameter compared to OEM. The speedometer error is noticeable and may warrant a recalibration if you rely on indicated speed. Expect a more planted steering feel, at the cost of some of the cushioning a taller sidewall provides. Diameter delta falls in the cautious 3–5% range, where speedometer recalibration and a careful clearance check are worth doing.
TakePractical direction for winter wheels, chains, or rougher pavement where cushioning matters.
Quick math: 255/40 R17 is 32.9 mm shorter than 235/45 R18, shifting the speedometer by -4.92%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
-32.9 mm
-4.92%
Speedometer at 100
95.1 km/h
-4.92% error
Ground clearance
-16.5 mm
ride height delta
Sidewall change
-3.8 mm
revs/km: 500.6
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/compare/235-45-r18-vs-255-40-r17Detailed comparison
| Metric | 235/45 R18 | 255/40 R17 | Difference |
|---|---|---|---|
| Overall diameter | 668.7 mm | 635.8 mm | -32.9 mm (-4.92%) |
| Sidewall height | 105.8 mm | 102.0 mm | -3.8 mm |
| Circumference | 2.101 m | 1.997 m | -103.4 mm |
| Revs / km | 476.0 | 500.6 | +24.6 |
| Ground clearance | reference | -16.5 mm | -16.5 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 95.1 km/h | -4.92 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 95.1 km/h after switching to 255/40 R17 — a -4.92% 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 -16.5 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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235/45 R18
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255/40 R17
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