Some would say it's a cultural thing: In the US, fuel efficiency is measured by how far a gallon of gas will get you, and the number (in mpg) rises with efficiency. In Europe, the higher the fuel efficiency, the smaller the number (in liters/100km). Perhaps it's something to do with the fact that gas is routinely 4 times more expensive in Europe than in the US. The goal is to design a car that will require a single liter to travel 100km, which amounts to 235mpg. Whew!
US (mpg) = 235 / EU (liters/100km)
1 mile = 1.609 km
1 US gallon = 3.785 liter
20 l/100km :: 11.8 mpg
10 l/100km :: 23.5 mpg
5 l/100km :: 47.0 mpg
3 l/100km :: 70.5 mpg
In Asia, they like to quote efficiencies in km/liter. To convert that to mpg, use:
US (mpg) = 2.35 * ASIA (km/liter)
When traveling UK sites, be aware that an imperial gallon is a bit more than a US gallon. So:
US (mpgUS) = 0.83 * IMP (mpgimp)
Of course, there are plenty of sites that will help you do the conversion, in case you don't have your handy calculator on you.