The formula with 3.5 substituted
US gallons = 3.5 ÷ 3.785411784 = 0.925
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.
3.5 litres is 0.925 US gallons and 0.770 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 3.5 litres means in practice
3.5 litres is 0.925 US gallons or 0.770 imperial gallons, and like 2.5 it is looked up mainly as an engine size: the 3.5 L V6 has powered a great many minivans, SUVs and pickups. It is a shade under a US gallon of displacement.