The formula with 100 substituted
litres = 100 × 3.785411784 = 378.541
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.
100 US gallons is 378.541 litres. 100 imperial gallons, the gallon behind British mpg figures, is 454.609 litres.
What 100 US gallons means in practice
100 US gallons is 378.541 litres; 100 imperial gallons is 454.609 litres. A hundred US gallons is the fuel in a large boat's tank or a farm's bulk store, and it weighs roughly 280 kg as petrol.