Comparison
235/40 R18 vs 255/40 R18
Stepping from 235/40 R18 to 255/40 R18 keeps the 18-inch wheel but widens the section by 20 mm. This sizing approach shifts overall diameter slightly from OEM. The wider section adds contact patch and lateral stability, while eating into fender and suspension clearance.
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.
TakeUseful when extra dry grip and stance matter more than a small fuel-economy hit.
Quick math: 255/40 R18 is 16.0 mm taller than 235/40 R18, shifting the speedometer by +2.48%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
+16.0 mm
2.48%
Speedometer at 100
102.5 km/h
+2.48% error
Ground clearance
+8.0 mm
ride height delta
Sidewall change
+8.0 mm
revs/km: 481.4
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/compare/235-40-r18-vs-255-40-r18Detailed comparison
| Metric | 235/40 R18 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 645.2 mm | 661.2 mm | +16.0 mm (+2.48%) |
| Sidewall height | 94.0 mm | 102.0 mm | +8.0 mm |
| Circumference | 2.027 m | 2.077 m | +50.3 mm |
| Revs / km | 493.4 | 481.4 | -11.9 |
| Ground clearance | reference | +8.0 mm | +8.0 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 102.5 km/h | +2.48 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 102.5 km/h after switching to 255/40 R18 — a +2.48% 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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235/40 R18
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255/40 R18
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