The chin spoiler really should not be any lower than the lowest thing on the underside which can grab air. My S-10 4x4 has been raised a little by a previous owner, and the chin is about 7-1/2" off the ground.
Chin Spoiler:
Chin Spolier - fast/easy/cheap/effective - Pelican Parts Technical BBS
If you have a cover on the bed, then yes air flow will trip over the top of the tailgate lip. On my older truck I re-directed it there on purpose with a roof wing. Your truck has an arc in the roof which does the same.
Rear Spoiler:
Rear Spoiler on Pick Up Truck - Experiment - Pelican Parts Technical BBS
Roof wing:
http://ecomodder.com/forum/showthrea...1-a-19525.html
From what I've reading and been seeing on pick-up truck wind tunnel pictures, they have addressed many of the issues you questioned. However, one of the wind tunnel pictures of the Ram has been said to be photo-shopped. I have no verification of that, it's just what another forum member claimed/observed.
http://s184.photobucket.com/user/kac...?sort=3&page=1
