Comparison
235/55 R17 vs 265/40 R18
Plus-sizing from 235/55 R17 to 265/40 R18 keeps overall diameter close to factory while opening room for a larger 18-inch wheel. This alternative fitment moves rolling diameter well outside the usual OEM tolerance. Indicated speed will drift far enough that recalibration is worth considering. Expect a more planted steering feel, at the cost of some of the cushioning a taller sidewall provides. The 3–5% diameter gap puts this in caution territory: doable on many cars, but verify clearance and consider recalibration.
TakeA solid pick for drivers chasing a more aggressive stance without abandoning OEM rolling diameter.
Quick math: 265/40 R18 is 21.1 mm shorter than 235/55 R17, shifting the speedometer by -3.06%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
-21.1 mm
-3.06%
Speedometer at 100
96.9 km/h
-3.06% error
Ground clearance
-10.5 mm
ride height delta
Sidewall change
-23.3 mm
revs/km: 475.7
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/compare/235-55-r17-vs-265-40-r18Detailed comparison
| Metric | 235/55 R17 | 265/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 690.3 mm | 669.2 mm | -21.1 mm (-3.06%) |
| Sidewall height | 129.3 mm | 106.0 mm | -23.3 mm |
| Circumference | 2.169 m | 2.102 m | -66.3 mm |
| Revs / km | 461.1 | 475.7 | +14.5 |
| Ground clearance | reference | -10.5 mm | -10.5 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 96.9 km/h | -3.06 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 96.9 km/h after switching to 265/40 R18 — a -3.06% 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 -10.5 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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235/55 R17
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265/40 R18
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