I have Vetter's DVD “How they got 470 MPG” and it says a whole lot. In a few words he would say that good aerodynamic forms are rounded in the front, pointed at the back and consist of smooth continuous contours. Trout shaped if you will. No heads, mirrors or tires (limited anyway) sticking out and upsetting the airflow. This combined with light weight takes very little HP to go 60+ MPH. Very little HP equals very little fuel burn.
A great deal of information and the DVD is worth the asking price.