Aerodynamics is the dominant factor for getting good gas mileage at highway speed. The faster you go the more important air drag is. While most of us think of the aerodynamic form of our vehicle as being fixed at the factory, that is actually not so. You can add aerodynamic extenders made out of fiberglass or even plywood or cardboard that can actually reduce drag considerably. For example, you can add a boat tail to your car in your own garage! The video shows how this can be done. In the video, a Geo Metro had a homemade boat tail added. This simple addition improved fuel economy 15% at highway speeds of 55 miles per hour.