The formula with 10 substituted
litres = 10 × 3.785411784 = 37.854
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.
10 US gallons is 37.854 litres. 10 imperial gallons, the gallon behind British mpg figures, is 45.461 litres.
What 10 US gallons means in practice
10 US gallons is 37.854 litres; 10 imperial gallons is 45.461 litres. The imperial figure was the tank size quoted in many British car handbooks before the 1990s, which is why a '10 gallon tank' on an old Mini holds 45 litres, not 38.