There is a web site that focuses on improving economy at www.ecomodder.com
, and they have an aerodynamics forum with some pretty knowledgeable people. Drag is drag, whether you want to apply an improvement to better fuel economy at fixed speed or to get more to speed at constant hp. One poster, screen name aerohead, has a T100 pickup that gets over 45 mpg; he's had it in a wind tunnel once and plans to go back. He highly recommends a textbook, Aerodynamics of Road Vehicles: From Fluid Mechanics to Vehicle Engineering ([Proceedings] / SAE), by Hucho, Wolf-Heinrich. Amazon has it for about $60, I've bought it but am only part way through it so far. I've learned a lot by lurking there (and here
). One useful sticky thread in the aero forum is a template based on a wing (there's an infinite number of wing length to cross section heights, they chose one that seems a good compromise to wind up with a practical vehicle shape and seems to be very close to what many top fuel mileage vehicles are based on). They include instructions but basically, take a picture of your car from the side from as far away as practical to minimize lens aberrations, then put it in a photo editing program and then overlay the template, keeping the aspect ratio fixed and scaling it so that it touches the ground at the bottom and touches the highest point on the roofline at the top. The closer your vehicle comes to that shape the lower the drag will be. One thing I was surprised to learn is how important the rear of he vehicle is. As long as the front isn't totally hateful, like a vertical sheet of plywood, you can pretty much ignore it and so long as you get the rear to match the template you will be surprisingly close to what an ideal overall shape would give.