The formula with 1 substituted
litres = 1 × 3.785411784 = 3.785
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.
1 US gallons is 3.785 litres. 1 imperial gallons, the gallon behind British mpg figures, is 4.546 litres.
What 1 US gallons means in practice
One US gallon is exactly 3.785411784 litres and one imperial gallon is exactly 4.54609 litres. Both figures are legal definitions, not measurements, and the 20% gap between them is the cause of most confusion about mpg figures between the US and the UK.