Its hardly a tough target.
From a BBC article
"A 2007 law required that new cars and trucks produced by 2020 obtain 35 miles per gallon of fuel."
35 mpg?! European small cars are achieving almost double that. My 1999 diesel even gets 60ish mpg.
Maybe if the American car giants imported a few efficient European engines, and started weaning Americans off this whole 6 litre engine stuff, the could make some progress.