Uany economist will tell you its the used car market that seriously hurts our fossil fuel numbers, currently the USA averages 14MPG and that is because of all the poor folks in used cars on the road.
No, its because of all the trucks, SUVs and otherwise inefficient cars on the road. I just bought a used 1996 Toyota Carolla with 230k miles on it. I checked my gas mileage (mixed city/freeway) at 26 MPG on my last fill up.
My friend who drives a 2008? BMW M3 told me gets 23 MPG.