Comparison
245/40 R20 vs 255/45 R18
Switching from 245/40 R20 to 255/45 R18 steps down to a 18-inch wheel — a familiar move for winter and dedicated all-terrain sets. This alternative fitment trims or stretches rolling diameter by a small margin. Expect a slight but noticeable shift in indicated speed compared to the original tires. More sidewall typically improves comfort and curb protection, especially on city streets. Overall the swap sits inside the safe ±3% diameter window, so ABS, traction control and gearing behave normally.
TakePractical direction for winter wheels, chains, or rougher pavement where cushioning matters.
Quick math: 255/45 R18 is 17.3 mm shorter than 245/40 R20, shifting the speedometer by -2.46%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
-17.3 mm
-2.46%
Speedometer at 100
97.5 km/h
-2.46% error
Ground clearance
-8.6 mm
ride height delta
Sidewall change
+16.8 mm
revs/km: 463.5
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/compare/245-40-r20-vs-255-45-r18Detailed comparison
| Metric | 245/40 R20 | 255/45 R18 | Difference |
|---|---|---|---|
| Overall diameter | 704.0 mm | 686.7 mm | -17.3 mm (-2.46%) |
| Sidewall height | 98.0 mm | 114.8 mm | +16.8 mm |
| Circumference | 2.212 m | 2.157 m | -54.3 mm |
| Revs / km | 452.1 | 463.5 | +11.4 |
| Ground clearance | reference | -8.6 mm | -8.6 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 97.5 km/h | -2.46 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 97.5 km/h after switching to 255/45 R18 — a -2.46% 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 -8.6 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 R20
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255/45 R18
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