chevy is really good at making aero cars..... stingray corvette, cleaned up bodywise I doubt you could do much better
[img[http://imagecache2.allposters.com/images/pic/DES/D694~Chevy-Corvette-Stingray-Posters.jpg[/img]
and there are a few 7th gen civic Si's running around with RSX-s transmissions which are 6speed and have a taller final drive, they get signficiantly better gas milage. one guy thew the type s final on his Si gears with a TSX 5th added as 6th (confused yet?) for much taller gears (he also has a bored and stroked with a LOT more power) and gets around 38mpg out of it. this vs my stock motor with almost 100less hp and stock gears at 10mpg less....