Comparison
255/40 R18 vs 275/40 R18
On the same 18-inch wheel, 275/40 R18 grows the section width by 20 mm versus 255/40 R18. This setup trims or stretches rolling diameter by a small margin. Extra width broadens the footprint for more grip, but check inner liner and strut clearance before fitting.
The speedometer offset is small but measurable; worth keeping in mind if you watch the dash closely. Diameter change stays inside the conservative ±3% safety window — an OEM-safe fitment on most vehicles.
TakeReasonable performance-leaning swap as long as fender and suspension clearance check out.
Quick math: 275/40 R18 is 16.0 mm taller than 255/40 R18, shifting the speedometer by +2.42%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
+16.0 mm
2.42%
Speedometer at 100
102.4 km/h
+2.42% error
Ground clearance
+8.0 mm
ride height delta
Sidewall change
+8.0 mm
revs/km: 470.0
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/compare/255-40-r18-vs-275-40-r18Detailed comparison
| Metric | 255/40 R18 | 275/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 661.2 mm | 677.2 mm | +16.0 mm (+2.42%) |
| Sidewall height | 102.0 mm | 110.0 mm | +8.0 mm |
| Circumference | 2.077 m | 2.127 m | +50.3 mm |
| Revs / km | 481.4 | 470.0 | -11.4 |
| Ground clearance | reference | +8.0 mm | +8.0 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 102.4 km/h | +2.42 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 102.4 km/h after switching to 275/40 R18 — a +2.42% 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 +8.0 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 R18
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275/40 R18
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