Comparison
245/45 R18 vs 255/40 R18
On the same 18-inch wheel, 255/40 R18 grows the section width by 10 mm versus 245/45 R18. This alternative fitment shifts overall diameter slightly from OEM. Less sidewall flex usually translates to crisper turn-in and a slightly stiffer ride over rough pavement.
Expect a slight but noticeable shift in indicated speed compared to the original tires. 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 16.5 mm shorter than 245/45 R18, shifting the speedometer by -2.43%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
-16.5 mm
-2.43%
Speedometer at 100
97.6 km/h
-2.43% error
Ground clearance
-8.3 mm
ride height delta
Sidewall change
-8.3 mm
revs/km: 481.4
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/compare/245-45-r18-vs-255-40-r18Detailed comparison
| Metric | 245/45 R18 | 255/40 R18 | Difference |
|---|---|---|---|
| Overall diameter | 677.7 mm | 661.2 mm | -16.5 mm (-2.43%) |
| Sidewall height | 110.3 mm | 102.0 mm | -8.3 mm |
| Circumference | 2.129 m | 2.077 m | -51.8 mm |
| Revs / km | 469.7 | 481.4 | +11.7 |
| Ground clearance | reference | -8.3 mm | -8.3 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 97.6 km/h | -2.43 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 97.6 km/h after switching to 255/40 R18 — a -2.43% 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 -8.3 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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245/45 R18
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255/40 R18
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