Comparison
215/40 R18 vs 255/40 R18
255/40 R18 is a wider variation of 215/40 R18 on the same 18-inch rim, adding 40 mm of tread footprint. This wheel and tire pairing moves rolling diameter well outside the usual OEM tolerance.
The dashboard speed will be significantly off — plan on recalibration before daily use. More tread on the ground tends to improve dry grip and stance, with a small fuel-economy and clearance tradeoff. A diameter change beyond 5% is aggressive enough to influence ABS, traction control and gearing; treat it as a serious modification.
TakeTreat as a serious modification — verify clearance, recalibrate the speedometer and reassess load capacity.
Quick math: 255/40 R18 is 32.0 mm taller than 215/40 R18, shifting the speedometer by +5.09%.
Current Tire
New Tire
Ground line · scaled comparison
Not Recommended
Over 5% — speedometer & ABS may misread
Diameter change
+32.0 mm
5.09%
Speedometer at 100
105.1 km/h
+5.09% error
Ground clearance
+16.0 mm
ride height delta
Sidewall change
+16.0 mm
revs/km: 481.4
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/compare/215-40-r18-vs-255-40-r18Detailed comparison
| Metric | 215/40 R18 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 629.2 mm | 661.2 mm | +32.0 mm (+5.09%) |
| Sidewall height | 86.0 mm | 102.0 mm | +16.0 mm |
| Circumference | 1.977 m | 2.077 m | +100.5 mm |
| Revs / km | 505.9 | 481.4 | -24.5 |
| Ground clearance | reference | +16.0 mm | +16.0 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 105.1 km/h | +5.09 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 105.1 km/h after switching to 255/40 R18 — a +5.09% 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 +16.0 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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215/40 R18
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255/40 R18
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