The study of Aerodynamics has brought forth all sorts of fancy designs, but the most basic aerodynamic shape is one that is smooth and has a small frontal surface area. If you look at anything fast - fighter jet, indy car, etc., they all have a very small frontal surface area. This reduces wind resistance, and allows for more speed.

I don't know about you folks, but I've stuck my hand outside of my car window while cruisin' down the highway and played with the wind. When you put your hand straight into the wind, there's very little resistance... and when you put your hand on end there is a heck of a lot of resistance.

Smoothness also counts. A good example is an Indycar - they are mostly streamlined, but they have all sorts of sticky-outy-bits. The driver's head, the wheels, the roll bar - if all of these things were removed (or encased in something more streamlined) the vehicle would be more aerodynamic. Why don't they do that then? The added weight of casing all that stuff in would do more harm than good. So even though aerodynamics are desireable, they aren't always practical.

For the most part, a sports car will get more mileage than a truck, because of the front end of a truck has so much wind resistance. If you've even been bike riding, roller blading, or ice skating... You know how much speed you gain if you just crouch yourself down a bit!