The formula with 5 substituted
US gallons = 5 ÷ 3.785411784 = 1.321
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 Litres to Gallons Converter explains where each comes from and gives a longer table.
5 litres is 1.321 US gallons and 1.100 imperial gallons. The imperial gallon is 4.54609 litres against the US gallon's 3.785411784, which is why the two answers differ by a fifth.
What 5 litres means in practice
5 litres is 1.321 US gallons or 1.100 imperial gallons, the size of the standard jerry can in much of Europe and of the engine in a great many American V8s. It is about a tenth of a typical car's fuel tank.