The formula with 2.5 substituted
US gallons = 2.5 ÷ 3.785411784 = 0.660
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.
2.5 litres is 0.660 US gallons and 0.550 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 2.5 litres means in practice
2.5 litres is 0.660 US gallons or 0.550 imperial gallons. The value is searched for constantly because 2.5 L is one of the most common engine sizes in the world, fitted to millions of sedans and crossovers, and readers want to know what that means in the units their fuel is sold in. The answer, about two thirds of a gallon, is the volume swept by the pistons in one cycle, and has nothing to do with how much fuel the engine uses.