Countries measure fuel economy in two opposite ways. The US and UK count distance per fuel, in miles per gallon, and Brazil, India and Japan use kilometers per liter; higher is better. Europe, Canada and Australia count fuel per distance, in liters per 100 km; lower is better. Because one is the reciprocal of the other, a car that improves from 20 to 25 mpg saves more fuel than one improving from 40 to 45. Note also that a UK gallon is 20% bigger than a US gallon, so UK mpg figures look higher.
Use the converter above for any pair. The most searched pairs have their own pages with worked examples and tables, listed above.