Comparison
245/40 R18 vs 255/40 R18
255/40 R18 is a wider variation of 245/40 R18 on the same 18-inch rim, adding 10 mm of tread footprint. This tire combination lands within OEM rolling-diameter tolerance. Speedometer drift stays small enough that most drivers won't notice it day to day. The wider section adds contact patch and lateral stability, while eating into fender and suspension clearance. Overall the swap sits inside the safe ±3% diameter window, so ABS, traction control and gearing behave normally.
TakeUseful when extra dry grip and stance matter more than a small fuel-economy hit.
Quick math: 255/40 R18 is 8.0 mm taller than 245/40 R18, shifting the speedometer by +1.22%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
+8.0 mm
1.22%
Speedometer at 100
101.2 km/h
+1.22% error
Ground clearance
+4.0 mm
ride height delta
Sidewall change
+4.0 mm
revs/km: 481.4
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/compare/245-40-r18-vs-255-40-r18Detailed comparison
| Metric | 245/40 R18 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 653.2 mm | 661.2 mm | +8.0 mm (+1.22%) |
| Sidewall height | 98.0 mm | 102.0 mm | +4.0 mm |
| Circumference | 2.052 m | 2.077 m | +25.1 mm |
| Revs / km | 487.3 | 481.4 | -5.9 |
| Ground clearance | reference | +4.0 mm | +4.0 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 101.2 km/h | +1.22 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 101.2 km/h after switching to 255/40 R18 — a +1.22% 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 +4.0 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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245/40 R18
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255/40 R18
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