In Frank Miller's comic and film "Sin City," the character Marv complains that modern cars all look like electric shavers. But there's a good reason the market shifted from land whales to sardines, and it's not just to gain a few MPG from weight loss. When it comes to fuel economy, aerodynamics is the name of the game. In a boxy Hummer, whether you roll with your windows down or up won't matter much, but you'll really feel the difference in a sleek new Nissan GT-R [source: Siler].
If rolling your windows down is so bad for gas mileage, why not run the AC? Well, you might want to -- but only at freeway speeds. Unlike your car's heater, which siphons free waste heat, air-conditioning actually sucks power from the engine and therefore burns more fuel. When the mercury tops 80 degrees F (27 degrees C), cutting down on AC can net you a fuel savings of 10-15 percent, so set your environmental control to vent and keep it there [sources: Allen; GSA; Wiesenfelder].
Of course, the same rules apply to turbulence-churning convertibles, sun roofs, moon roofs and accessories. In addition, nix those roof racks -- they can eat into your miles per gallon to the tune of 5 percent -- and skip aftermarket bug and wind deflectors [sources: Car Talk; Wiesenfelder; Yetiv].