I live in Canada where we typically get both mpg and liter/100 km. I've found that, almost universally, my friends that report mpg have less fuel efficient cars than the ones that quote liters per 100 km when asked about a vehicle's fuel efficiency.

I tend to think that one of the reasons that Americans tend to have less fuel efficient cars is that when they are evaluating alternatives 9 mpg and 10 mpg are "closer" or more equivalent than 47 mpg and 50 mpg. For example, if I go out and buy a Truck in Canada I could either compare

Ford F150: 15.7 liters/100km (15 mpg) city, 11.2 liters/100km (21 mpg) highway.

GMC Sierra 2500 4WD: 14.7 liters/100km (16 mpg) city, 11.76 liters/100km (20 mpg) highway.

With the Sierra, you are using a full liter less of gas during city driving at the cost of only half a liter of highway driving, so unless I'm doing more than 66% of my driving on highway like conditions, I should go with the Sierra, but that information does not come across to gas conscious buyers that use mpg.