I don't understand why 40 mpg isn't more common already. Without ANY aero mods, just driving habits, I've gotten 40 mpg from the following cars over the years. My first was an '86 Chevette loaded with camping gear for a long trip. And that 6yo rear drive puppy was closer to dead than new. My more recent 40+ achievers included '00 Prizm and '03 Spectra. All were 4dr models BTW which may be a plus in hindsight. Our 1995 Metro 2dr 3-cyl got 50 mpg (+2/-10) without any mods or any attention to driving for that matter. It was so consistent I thought the name was short for metronome.
Cheers
KB