The formula with 70 substituted
US gallons = 70 ÷ 3.785411784 = 18.492
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.
70 litres is 18.492 US gallons and 15.398 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 70 litres means in practice
70 litres is 18.492 US gallons or 15.398 imperial gallons, the tank of a large estate or a mid-size SUV. At 8 L/100km that is 875 km of range, about 544 miles.