The formula with 50 substituted
litres = 50 × 3.785411784 = 189.271
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.
50 US gallons is 189.271 litres. 50 imperial gallons, the gallon behind British mpg figures, is 227.305 litres.
What 50 US gallons means in practice
50 US gallons is 189.271 litres and 50 imperial gallons is 227.305 litres, the size of the drums fuel used to be delivered in and of the largest auxiliary tanks on long-haul pickups.