First of all, it's important to note that the way a driver drives has great influence on the fuel economy of the car. That, and making sure the tire pressure, engine/transmission fluids, and air filters are all maintained and clean. That said, I'm wondering what ideas people have for making a car a good deal more fuel efficient. I'm thinking along the lines of more crazy mechanical trickery then simply maintaining the car correctly and driving nice and granny-like. And so, sci-forum gear-heads, I ask thee to go nuts here. What could one mechanically do to a car to make it more fuel efficient without severely sacrificing it's utility (cutting hunks of the ceiling out or something). A few ideas I had... #1: Put a performance-grade oversized air filter. Sounded like a good idea, and would improve power output, however from what I can tell from online reports, fuel economy is not improved very much, maybe a percent or two. This might because while a bigger filter would allow more air into the engine, it doesn't remove the fact that it doesn't take too much of the engine's power to begin with to suck air through the filter in the first place. #2: Undersize the wheels and lower the suspension. Lower to the ground, a car should have a little less wind resistance, while smaller wheels at higher pressure put less rubber on the road, reducing rolling friction. #3: A big freaking tappered cone at the end of a car with a non-aerodynamic tail-end. Not exactly what you'd think of as a sexy car... but hey, what can you do... #4: Bolt an electric motor somewhere in the drive-train, make a coarse hybrid system of some kind. I dunno, go crazy, think of somethin!