Comparison
245/40 R20 vs 245/45 R18
Going from 245/40 R20 to 245/45 R18 is a minus-2 setup that adds sidewall on a smaller 18-inch wheel. This alternative fitment swings rolling diameter far enough to feel on the road. The speedometer error is noticeable and may warrant a recalibration if you rely on indicated speed. Extra sidewall absorbs impacts more readily — a sensible bias for daily commuting and broken pavement. The 3–5% diameter gap puts this in caution territory: doable on many cars, but verify clearance and consider recalibration.
TakePractical direction for winter wheels, chains, or rougher pavement where cushioning matters.
Quick math: 245/45 R18 is 26.3 mm shorter than 245/40 R20, shifting the speedometer by -3.74%.
Current Tire
New Tire
Ground line · scaled comparison
Slight Difference
Within ±5% — usable, recalibration recommended
Diameter change
-26.3 mm
-3.74%
Speedometer at 100
96.3 km/h
-3.74% error
Ground clearance
-13.1 mm
ride height delta
Sidewall change
+12.3 mm
revs/km: 469.7
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/compare/245-40-r20-vs-245-45-r18Detailed comparison
| Metric | 245/40 R20 | 245/45 R18 | Difference |
|---|---|---|---|
| Overall diameter | 704.0 mm | 677.7 mm | -26.3 mm (-3.74%) |
| Sidewall height | 98.0 mm | 110.3 mm | +12.3 mm |
| Circumference | 2.212 m | 2.129 m | -82.6 mm |
| Revs / km | 452.1 | 469.7 | +17.5 |
| Ground clearance | reference | -13.1 mm | -13.1 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 96.3 km/h | -3.74 km/h |
Verdict: warning
Between 3% and 5% — noticeable speedometer drift; recalibration may be advisable.
Speedometer impact
At a true 100 km/h, your dashboard will read 96.3 km/h after switching to 245/45 R18 — a -3.74% 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 -13.1 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.
Back to
245/40 R20
Back to
245/45 R18
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