Comparison
245/40 R18 vs 255/45 R18
Stepping from 245/40 R18 to 255/45 R18 keeps the 18-inch wheel but widens the section by 10 mm. This tire combination moves rolling diameter well outside the usual OEM tolerance. Extra sidewall absorbs impacts more readily — a sensible bias for daily commuting and broken pavement.
The dashboard speed will be significantly off — plan on recalibration before daily use. A diameter change beyond 5% is aggressive enough to influence ABS, traction control and gearing; treat it as a serious modification.
TakeRecommended only after a professional fitment check and speedometer recalibration.
Quick math: 255/45 R18 is 33.5 mm taller than 245/40 R18, shifting the speedometer by +5.13%.
Current Tire
New Tire
Ground line · scaled comparison
Not Recommended
Over 5% — speedometer & ABS may misread
Diameter change
+33.5 mm
5.13%
Speedometer at 100
105.1 km/h
+5.13% error
Ground clearance
+16.8 mm
ride height delta
Sidewall change
+16.8 mm
revs/km: 463.5
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/compare/245-40-r18-vs-255-45-r18Detailed comparison
| Metric | 245/40 R18 | 255/45 R18 | Difference |
|---|---|---|---|
| Overall diameter | 653.2 mm | 686.7 mm | +33.5 mm (+5.13%) |
| Sidewall height | 98.0 mm | 114.8 mm | +16.8 mm |
| Circumference | 2.052 m | 2.157 m | +105.2 mm |
| Revs / km | 487.3 | 463.5 | -23.8 |
| Ground clearance | reference | +16.8 mm | +16.8 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 105.1 km/h | +5.13 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 255/45 R18 — a +5.13% 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 +16.8 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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255/45 R18
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