A study completed earlier this year by the French environment and energy agency Ademe and the World Energy Council indicates that over
nearly 30 years, the average fuel consumption of European cars has
dropped by more than 20% to around 6.5 l/100km, or 43 mpg. By contrast, average fuel economy of new cars in the US is now
21 29.3 mpg. (See update below.)
France itself appears to be the European fuel economy leader, with the average consumption of new cars on sale in France now at 46 mpg.
Contributing factors are the ongoing shift to diesel, the improved efficiency of the vehicles, public policies maintaining high fuel prices to discourage consumption, and a strong focus on public transportation.
Update: I may have been too hasty with my earlier calculations. If the Ademe figures refer just to cars (as John asks below...and which I'm trying to confirm one way or the other), then the picture is slightly different.
According to The National Highway Safety Traffic Safety Administration, while the CAFE standard for 2004 passenger cars was 27.5 mpg, the actual composition of the models in the US passenger car fleet yielded an average 29.3 mpg. The average fuel economy of the entire 2004 model year fleet (cars and light duty trucks and SUVs) was 24.7 mpg.
Using that as a baseline, then the fuel economy of the average US new car is 32% worse than that of Europe. Or, to put it the other way, European passenger car fuel economy is 47% better than that of the US.