I like to say that the two biggest things you can do to improve your fuel economy are learn to drive and modify your aerodynamics. Drag reduction takes a back seat to driving techniques, but no one is saying you can't do both! Hopefully this will serve as an overview of basic sources of drag and the modifications that people go through to try and cut their Coefficient of Drag to more fuel economy-friendly levels.
Before we get into the sources and solutions for drag, check out this equation that describes the makeup of aerodynamic drag in a passenger vehicle:
* V = wind velocity in tunnel
* S = frontal area of vehicle
* E = wheelbase
* Cx = coefficient of drag
* Cy = coefficient of drift
* Cz = coefficient of lift
Now that you've seen that, keep it in mind, and on to the good stuff:
General Shape -
The shape of the car is the most important thing to aerodynamics. While you can't do much about your existing car, you can consider aerodynamics when purchasing your car. Most cars, such as the CRX, will have Coefficient of Drag (Cd) statistics readily available to help in your decision making process. Just so everyone knows, the second generation CRX Si and DX had a Cd of .30 and the HF came in at .29. The difference here is due to the HF's lack of a rear wiper.
As I said, there's not much you can do about general shape, but if you're up for it, there is one thing. This modification is called a boattail. One of the main principles in aerodynamic body shape is that a smooth, long, tapered surface should be attempted in order to keep attached flow through the end of the car. But, at the end flow is generally cut off abruptly, which is where a boat tail, a short extension of then end of the car, adds a greater length over which flow may taper. Though this isn't done much, some great motivation for attached flow can be taken from Phil Knox's modified truck.
Mirrors -
Mirrors contribute a huge amount to the aerodynamic drag of a car, especially for how small they are. There are several solutions to this problem. One such solution is to install smaller mirrors that won't create such a large amount of frontal area and turbulence, as seen here:
Another solution is to install folding mirrors (if you don't already have them) and fold the mirrors back when you reach highway speeds. Also, you could just eliminate the passenger mirror, or both, using head turns to check before making lanes changes and performing other maneuvers. Many Insight drivers have eliminated their mirrors and replaced them with cameras so that they could still get an optimal vantage point without turning their heads. One note of caution: If you're considering removing or altering your mirrors do a test drive before making the change permanent; you don't want to get in over your head safety-wise.
Underbody -
The underbody is another big source of drag on most vehicles. Most underbodies are designed without care for aerodynamics, which is good and bad. Bad because most of us are starting off with terrible underbodies, good because there is a great improvement in Cd to be had from a full undertray. Check out Dan's DIY on the undertray here. Some cars, such as the Acura NSX come with full undertrays, but for the rest of us, "The contribution can be up to 25% of the total drag. Careful design can achieve a change in Cd of -0.02 on serial production cars." Check out this Chalmers report as the source and for more information on underbodies and drag.
Cooling -
Particularly because of the shape and design of the radiator it causes a large amount of aerodynamic drag. Radiators are designed for worse case scenarios, scenarios most of us are never likely to be in, which is why a grill/radiator block can easily and safely be installed on most cars. Check out this article detailing my grill block installation to get a few ideas on how to do this. If you're considering modifying your car for improved aerodynamics and fuel economy, this is the first place for you to start!
Tires -
Surprising, tires cause a large amount of aerodynamic drag. This drag can be reduced by moving to a slimmer tire and reducing area in the wheel well where turbulent flow can exist. Check out this thread for a great DIY on installing some rear wheel skirts. While that probably should not be discussed here, one of the main issues with wheels is that they cause a huge amount of turbulence, and if you can block air from getting to them that turbulence will be greatly reduced.
Deserving of its own paragraph, wheel covers are a stylish and cool way to modify your tires for fuel economy. Lacking a picture of some mounted on a car at the moment, check out this set on the right to see what you're getting yourself into: They do work and your brakes will still cool themselves properly, so don't worry about safety with these things. Happy hubcapping!
Unfortunately, I forgot to mention wheel spoilers. These are relatively simple things, designed to gently divert air under the car from ramming directly into the tires. They will not do much in terms of drag reduction, but the benefit is there, so keep them in mind when building an undertray.
Wipers -
As I noted previously, the rear wiper on the CRX makes a .01 difference in Cd. Consider removing it and treating your rear window with a product such as Rain-X. However, as I mentioned earlier with the mirrors, go out for a rainy drive without using your rear wiper before you decide to eliminate it completely. Some eliminate one of both of their front wipers and rely on glass treatments, but this is not something I could do. I will, however, endorse the idea that you remove the blades and keep them in the trunk, so that if it does rain you can simply pull over and pop them back on!
Misc. -
The radio antenna rounds of the rest of the serious drag contributions. Check out my writeup here to check out my antenna removal.
Final Notes -
Aerodynamic modifications are the most important modifications you can make to your car. Take them seriously and you can have a good looking car with greatly improved fuel economy. Good luck and send me some pictures when you're done!