Brad Plumer:

The fuel economy of new cars and trucks in the United States basically stagnated during the 1980s and ’90s before rising again after 2004.
 Why the sudden shift? For one, global oil prices started rising, spurring drivers to buy fuel-efficient vehicles. Then, in 2007, Congress imposed a new round of fuel-economy standards on automakers — the first time those requirements had been tightened since 1984. The Obama administration later raised those fuel-economy goals even higher.
 (Also note that this is only for new cars sold in a given year. The average fuel economy of all cars on the road is obviously lower: about 23.3 miles per gallon in 2012 for light-duty vehicles with a short wheel base, and 17.1 miles per gallon for vehicles with a longer wheel base.)