Comparison
215/55 R17 vs 255/40 R18
Switching from 215/55 R17 to 255/40 R18 is a plus-1 upgrade that wraps a shorter sidewall around a larger 18-inch wheel. This sizing approach preserves rolling diameter within a hair of the original.
Speedometer drift stays small enough that most drivers won't notice it day to day. Expect a more planted steering feel, at the cost of some of the cushioning a taller sidewall provides. The wider section adds contact patch and lateral stability, while eating into fender and suspension clearance. Many drivers pick this direction primarily for appearance — the bigger rim simply looks more aggressive. Diameter change stays inside the conservative ±3% safety window — an OEM-safe fitment on most vehicles.
TakeA solid pick for drivers chasing a more aggressive stance without abandoning OEM rolling diameter.
Quick math: 255/40 R18 is 7.1 mm shorter than 215/55 R17, shifting the speedometer by -1.06%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
-7.1 mm
-1.06%
Speedometer at 100
98.9 km/h
-1.06% error
Ground clearance
-3.5 mm
ride height delta
Sidewall change
-16.3 mm
revs/km: 481.4
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/compare/215-55-r17-vs-255-40-r18Detailed comparison
| Metric | 215/55 R17 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 668.3 mm | 661.2 mm | -7.1 mm (-1.06%) |
| Sidewall height | 118.3 mm | 102.0 mm | -16.3 mm |
| Circumference | 2.100 m | 2.077 m | -22.3 mm |
| Revs / km | 476.3 | 481.4 | +5.1 |
| Ground clearance | reference | -3.5 mm | -3.5 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 98.9 km/h | -1.06 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 98.9 km/h after switching to 255/40 R18 — a -1.06% 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 -3.5 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.
Back to
215/55 R17
Back to
255/40 R18
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