The formula with 20 substituted
km/L = 20 × 0.425144 = 8.50
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The MPG to km/L Converter explains where each comes from and gives a longer table.
The same 20 mpg (US) is 24.02 mpg on the imperial gallon, 11.76 L/100km and 8.50 km/L. If your 20 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 7.08 km/L instead.
What 20 mpg (US) means in practice
20 mpg (US) is 8.50 km/L, the efficiency of a large SUV or a pickup. To a reader in India or Japan, 8.5 km/L is a figure associated with big petrol engines and a heavy right foot.