Aerodynamics is the study of moving gases (in this case air) over a body in motion, and how that airflow will affect the body's movement through the flow. Car aerodynamics is used to make passenger vehicles safer and more fuel efficient, but it is primarily used in race car design where high speeds create an even greater need for specialized designs.
In many, if not most, of the passenger vehicles that people see on the road, aerodynamics plays a comparatively minor role in the overall design of the vehicle. Aside from making a car as aerodynamically safe as possible within specific design parameters, other considerations arguably play more prominently into the final production line of most vehicles.
One demand that takes precedence over auto aerodynamics is the public's demand for interior space. Large SUVs are very convenient for families, vacations, and grocery lugging, but the necessary box-like design brings with it certain compromises. Simply put, the bulkier or less streamlined a vehicle is, the greater force will be exerted upon it from air-drag as it pushes against the air during acceleration. All things being equal, this cuts into fuel efficiency. It also translates to comparatively poor handling at high speeds when, in the case of an emergency, a sharp turn of the wheel is required to avoid hitting an on-coming object. This is not to say that SUVs are not safe, but that they are less safe in certain situations than those vehicles that are designed based primarily on aerodynamics.
Examples of vehicles based primarily on car aerodynamics are those that lie low to the ground, are sleek in design, and have rounded lines with reclining windshields that allow the air to easily flow over and around them, rather than "butt up" against flat or vertical surfaces. In a word, sports cars. Aside from being smaller and therefore lighter than their SUV counterparts, these low-profile cars don't encounter the wind resistance of SUVs or trucks, and therefore get better fuel efficiency. The trade-off is that they have less interior room, and some would argue that, although they handle better, if the driver does end up in an accident, he or she might prefer to have the protection of a larger surrounding car body.
In sedans, automakers attempt to strike a balance between interior room, aerodynamics, and handling. For many years, sedans also had something other cars didn't: luxury lines providing rich interiors. As the demand for recreational vehicles grew, however, automakers endowed them with the same luxury features, causing many families to prefer the car with more versatility, if not better mileage.
Although aerodynamics continues to play an important role in car design, it is most important in race car design. The same principles that apply to aircraft are applied here, but with a twist. The shape of a car chassis is similar to an inverted wing or airfoil that creates downforce rather than lift. This keeps the car on the ground at high speeds and increases traction around curves because the air under the car is moving faster than that the air above it. Slower moving air creates greater pressure, forcing the car down against the track. The shape of the underbody, which has another inverted wing, creates another area of low pressure that sucks the car to the asphalt.
Downforce must be balanced against drag, which slows the car. Designs are refined through research using wind tunnels equipped with moving tracks to simulate racing conditions. Efficient compromise of the downforce/drag relationship for the best handling at the highest possible speeds is the goal. When applicable, understanding gained in race car aerodynamics is often applied to make passenger vehicles safer and more efficient.
With any passenger car design, there will be compromises in some areas. Weighing these compromises against personal needs and tastes results in the variety of popular cars seen on the road today; the understanding of how the car moves through the surrounding air improves handling and helps to make every car more fuel-efficient.