Comparison
245/45 R20 vs 255/55 R18
Going from 245/45 R20 to 255/55 R18 is a minus-2 setup that adds sidewall on a smaller 18-inch wheel. This wheel and tire pairing preserves rolling diameter within a hair of the original.
The speedometer offset is mild and well inside what most cars can tolerate without recalibration. Extra sidewall absorbs impacts more readily — a sensible bias for daily commuting and broken pavement. More tread on the ground tends to improve dry grip and stance, with a small fuel-economy and clearance tradeoff. Minus-sizing keeps replacement costs down and opens up a wider range of winter and all-terrain tires. Diameter change stays inside the conservative ±3% safety window — an OEM-safe fitment on most vehicles.
TakePractical direction for winter wheels, chains, or rougher pavement where cushioning matters.
Quick math: 255/55 R18 is 9.2 mm taller than 245/45 R20, shifting the speedometer by +1.26%.
Current Tire
New Tire
Ground line · scaled comparison
Excellent Fit
Within ±3% — safe for daily driving
Diameter change
+9.2 mm
1.26%
Speedometer at 100
101.3 km/h
+1.26% error
Ground clearance
+4.6 mm
ride height delta
Sidewall change
+30.0 mm
revs/km: 431.5
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/compare/245-45-r20-vs-255-55-r18Detailed comparison
| Metric | 245/45 R20 | 255/55 R18 | Difference |
|---|---|---|---|
| Overall diameter | 728.5 mm | 737.7 mm | +9.2 mm (+1.26%) |
| Sidewall height | 110.3 mm | 140.3 mm | +30.0 mm |
| Circumference | 2.289 m | 2.318 m | +28.9 mm |
| Revs / km | 436.9 | 431.5 | -5.4 |
| Ground clearance | reference | +4.6 mm | +4.6 mm |
| Speedometer @ 100 km/h | 100.0 km/h | 101.3 km/h | +1.26 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 101.3 km/h after switching to 255/55 R18 — a +1.26% 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 +4.6 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.
Back to
245/45 R20
Back to
255/55 R18
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