The formula with 30 substituted
litres = 30 × 3.785411784 = 113.562
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.
30 US gallons is 113.562 litres. 30 imperial gallons, the gallon behind British mpg figures, is 136.383 litres.
What 30 US gallons means in practice
30 US gallons is 113.562 litres and 30 imperial gallons is 136.383 litres. Thirty US gallons is the fuel capacity of a large motorhome or a heavy-duty truck, and about 84 kg of petrol.