In more detail
There are two families of units: distance per fuel (miles per gallon, km per litre), where higher is better, and fuel per distance (litres per 100 km), where lower is better. L/100 km = 235.215 / US mpg, or 282.481 / UK mpg, because the imperial gallon (4.546 L) is larger than the US gallon (3.785 L): 30 US mpg is 36.0 UK mpg or 12.75 km/L. Official ratings come from standardised test cycles; real-world figures are usually lower, especially at motorway speeds, in cold weather and on short trips.
Why it matters
Because mpg is the reciprocal of fuel use, the same mpg gain saves far more fuel on a thirsty vehicle than on an efficient one, the so-called "mpg illusion". Fuel per distance makes running costs easy to compare and multiply.
Example
Fuel used over 100 miles:
10 mpg → 10.0 gal 20 mpg → 5.0 gal saves 5.0 gal
30 mpg → 3.33 gal 50 mpg → 2.0 gal saves 1.33 gal
30 mpg = 235.215 / 30 = 7.84 L/100 km
300-mile trip at 30 mpg = 10 gal → $35.00 at $3.50 a gallon
Going from 10 to 20 mpg saves almost four times as much fuel as going from 30 to 50.