The formula with 40 substituted
litres = 40 × 3.785411784 = 151.416
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.
40 US gallons is 151.416 litres. 40 imperial gallons, the gallon behind British mpg figures, is 181.844 litres.
What 40 US gallons means in practice
40 US gallons is 151.416 litres; 40 imperial gallons is 181.844 litres. Searched for often, probably because 40 gallons is the capacity of a common size of domestic water heater and of the auxiliary tanks fitted to work trucks. It is also about the fuel a 25 mpg car burns in 1,000 miles.