Comparison
215/40 R18 vs 255/40 R17
Minus-sizing from 215/40 R18 to 255/40 R17 pairs a smaller 17-inch wheel with more rubber between the rim and road. This swap preserves rolling diameter within a hair of the original.
The speedometer offset is mild and well inside what most cars can tolerate without recalibration. The wider section adds contact patch and lateral stability, while eating into fender and suspension clearance. The smaller wheel is also lighter and easier to find affordable winter rubber for. Diameter change stays inside the conservative ±3% safety window — an OEM-safe fitment on most vehicles.
TakePractical direction for winter wheels, chains, or rougher pavement where cushioning matters.
Quick math: 255/40 R17 is 6.6 mm taller than 215/40 R18, shifting the speedometer by +1.05%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
+6.6 mm
1.05%
Speedometer at 100
101.0 km/h
+1.05% error
Ground clearance
+3.3 mm
ride height delta
Sidewall change
+16.0 mm
revs/km: 500.6
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/compare/215-40-r18-vs-255-40-r17Detailed comparison
| Metric | 215/40 R18 | 255/40 R17 | Difference |
|---|---|---|---|
| Overall diameter | 629.2 mm | 635.8 mm | +6.6 mm (+1.05%) |
| Sidewall height | 86.0 mm | 102.0 mm | +16.0 mm |
| Circumference | 1.977 m | 1.997 m | +20.7 mm |
| Revs / km | 505.9 | 500.6 | -5.3 |
| Ground clearance | reference | +3.3 mm | +3.3 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 101.0 km/h | +1.05 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 101.0 km/h after switching to 255/40 R17 — a +1.05% 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 +3.3 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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215/40 R18
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255/40 R17
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