on a 1995 Ram ClubCab with a 6.5 foot long cargo box i got a 1 mpg increase on the highway with a hard ARE tonneau
there was no increase in city driving
US Patents are sworn to "under oath"
GM swore under oath that the best aerodynamics comes when you cover about half the open bed from the tailgate forward.
See this patent
http://www.google.com/patents/US4573730?...ved=0CDUQ6AEwAAthis drawing and graph shows the effect
http://patentimages.storage.googleapis.com/pages/US4573730-1.pngthere is also a good study of tonneaus on full sized GM and Ford pickups in the large wind tunnel owned by the Research Council of Canada, but unfortunately they did not include a Ram pickup
