I dunno, it seems like modern cars are heavier than old ones. At least similarly sized. I bet if you compared an old Dart or Valiant to a modern mid-size car, the modern one would be a couple hundred pounds heavier. I say if you want mileage, get a slant-6, a factory Jeep straight-6 EFI setup (or not, might be a bit tough to adapt, I don't know), and a turbo from the 80's turbo-4 cars. It would have a good amount of power and the mileage would hardly be any worse than an NA slant-6.
