The formula with 100 substituted
US gallons = 100 ÷ 3.785411784 = 26.417
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.
100 litres is 26.417 US gallons and 21.997 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 100 litres means in practice
100 litres is 26.417 US gallons or 21.997 imperial gallons, the size of a long-range tank on a large pickup or a 4x4 built for touring. It weighs about 75 kg as petrol and 84 kg as diesel.