Back in the late 70's I drove in a Peugeot 404 Utility Pick-up from
N'Djamena, Chad to Maidugiri, Nigeria to collect a boat. We strapped the boat
on top with the bow over the cab and the stern over the tailgate. 
On the return trip we got almost 40% better mpg.
Can you say "Kamm-back Pick-em-up"
Friends have reported similar, but less drastic improvement in mpg with a
camper shell installed.
JoE