From Winning the Oil Endgame (free login required):

The average new U.S. light vehicle in 2003 had 24% more weight, 93% more horsepower, and 29% faster 0–60-mph time than in 1981, but only 1% more miles per gallon. If 1981 performance had instead stayed constant, light vehicles would have become 33% more efficient

It's not a bad read, but it is 300+ pages of dense work.