Just a quickie,based on T-100/Dodge research
*Flatbed --------------------------------------- Cd 0.668
*Suburban type roof (Lear cap)------------------ Cd 0.39
*Pickup bed (open)------------------------------Cd 0.44
*Full tonneau ---------------------------------- Cd 0.415
*GM' Half-tonneau ------------------------------Cd 0.357
*Texas Tech (GM) Aero Shell ------------------- Cd 0.352
*ITworks double-geometry shell ---------------- Cd 0.325
*Spirit of Ecomodder.com----------------------- Cd 0.268-0.267
*Spirit of Ecomodder.com with folded mirrors----- Cd 0.251
*Spirit of Ecomodder.com with mirror delete (est.) Cd 0.24
*Complete boat-tail on custom truck box--------- Cd 0.12?
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Here's a semi-recent photo of a pickup chassis (Chrysler) illustrating the 'body-on-frame' nature of most trucks.The Ridgeline,VW,Subaru Brat, Dodge Rampage,etc. would be exceptions
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Here's the all-new for 1986 GM pickup revealing the deep chassis of which the airdam or belly pan will help with,drag-wise
Here is a shot of the blistered half-tonneau cover.It's the rear half of a plan-tapered aeroshell
Here is the blistered half-tonneau in combination with a very rudimentary cab-wing
Here is a shot which shows the aft-body more in true length
Here is a tuft study of the aft-body near home back in the day
