The formula with 15 substituted
litres = 15 × 3.785411784 = 56.781
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.
15 US gallons is 56.781 litres. 15 imperial gallons, the gallon behind British mpg figures, is 68.191 litres.
What 15 US gallons means in practice
15 US gallons is 56.781 litres, a common mid-size sedan tank; 15 imperial gallons is 68.191 litres. A driver who fills a 15 gallon tank at 30 mpg has bought 450 miles of range.