Comparison
255/40 R17 vs 265/40 R18
265/40 R18 is a plus-1 alternative to 255/40 R17 — the bigger wheel shows through a thinner sidewall. This sizing approach moves rolling diameter well outside the usual OEM tolerance. Speedometer error is large enough that recalibration is effectively required for accurate readings. Extra width broadens the footprint for more grip, but check inner liner and strut clearance before fitting. A diameter change beyond 5% is aggressive enough to influence ABS, traction control and gearing; treat it as a serious modification.
TakeRecommended only after a professional fitment check and speedometer recalibration.
Quick math: 265/40 R18 is 33.4 mm taller than 255/40 R17, shifting the speedometer by +5.25%.
Current Tire
New Tire
Ground line · scaled comparison
Not Recommended
Over 5% — speedometer & ABS may misread
Diameter change
+33.4 mm
5.25%
Speedometer at 100
105.3 km/h
+5.25% error
Ground clearance
+16.7 mm
ride height delta
Sidewall change
+4.0 mm
revs/km: 475.7
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/compare/255-40-r17-vs-265-40-r18Detailed comparison
| Metric | 255/40 R17 | 265/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 635.8 mm | 669.2 mm | +33.4 mm (+5.25%) |
| Sidewall height | 102.0 mm | 106.0 mm | +4.0 mm |
| Circumference | 1.997 m | 2.102 m | +104.9 mm |
| Revs / km | 500.6 | 475.7 | -25.0 |
| Ground clearance | reference | +16.7 mm | +16.7 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 105.3 km/h | +5.25 km/h |
Verdict: danger
Over 5% diameter difference — likely to affect speedometer accuracy, ABS calibration and gearing. Not recommended without professional review.
Speedometer impact
At a true 100 km/h, your dashboard will read 105.3 km/h after switching to 265/40 R18 — a +5.25% 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 +16.7 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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255/40 R17
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265/40 R18
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