54 mpg = 4.356 L/100km
There is one milligram in one milligram.
This is another question asking us to compare units of different quantities. Milligram is a unit of mass (or weight), and gallon is a unit of capacity (volume). There is no answer. You might as well ask how many ounces are in an inch.
13.5 gal 1 gallon = 4 quarts 1 quart = 0.25 gallon
There are 0.000001 kilograms in a milligram.
1,000mcg per milligram.