The formula with 12 substituted
litres = 12 × 3.785411784 = 45.425
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.
12 US gallons is 45.425 litres. 12 imperial gallons, the gallon behind British mpg figures, is 54.553 litres.
What 12 US gallons means in practice
12 US gallons is 45.425 litres, the tank capacity of a typical compact sedan sold in America; 12 imperial gallons is 54.553 litres. The compact's tank, in other words, is about the same size as a European hatchback's 45 litre one.