The formula with 5 substituted
litres = 5 × 3.785411784 = 18.927
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.
5 US gallons is 18.927 litres. 5 imperial gallons, the gallon behind British mpg figures, is 22.730 litres.
What 5 US gallons means in practice
5 US gallons is 18.927 litres, the size of the standard American gas can; 5 imperial gallons is 22.730 litres. A five gallon can is also roughly the fuel a 25 mpg car burns in 125 miles.