It seems the geometry of a truck bed has a great effect on the Cd of the pickup truck. This research paper presents some interesting wind tunnel data and explains why later model pickup trucks have such high side walls and tailgates.
It seems the manufacturers have sacrificed the utilitarian aspects of earlier model pickups (with low side walls and tailgates) for the taller bed height we see on the late model pickup trucks, especially those trucks with the shorter beds. It's all about reducing the Cd of the pickup truck to improve the fuel efficiency of the truck.
http://www.wseas.us/e-library/confer.../FMA/FMA17.pdf
Happy New Year and may 2011 be a good one for us all.
Bondo