There's a simple yet oft-forgotten law of physics that says the faster an object moves, the higher the pressure against its movement is - that is, more energy is needed to counteract this air pressure in order to maintain and increase speed.
This was a rather new field when Chrysler developed the Airflow models in the early thirties. The cars were fully designed following aerodynamics and tested in a wind tunnel. Despite the commercial efforts, such as the video you can watch if you follow the Read link, the car was a failure because the global result turned out to be odd for the average consumer.
Nevertheless, the cars could make 18 mpg with an I-8 engine, whereas most of the cars at that time barely could make 8-10. This was a lesson later applied to most of the cars and gave such groundbreaking models as the Citroën DS or the Studebaker Avanti. Almost every car made today is wind-tunnel tested to improve efficiency.
[Source: Greatcarstv.com, h/t to Chris]