The formula with 20 substituted
litres = 20 × 3.785411784 = 75.708
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 Gallons to Litres Converter explains where each comes from and gives a longer table.
20 US gallons is 75.708 litres. 20 imperial gallons, the gallon behind British mpg figures, is 90.922 litres.
What 20 US gallons means in practice
20 US gallons is 75.708 litres; 20 imperial gallons is 90.922 litres. Twenty US gallons is the tank of a large SUV, and at 18 mpg it is 360 miles of range, or 580 km.