Welcome to Ecomodder.
I created this for a thread here:
Replacing Class c motorhome body for touring act
It was modeled on 1930s beer trucks.
Also, consider the pop-top/aero combination here:
ennored -- I don't have a citation, but IIRC it's expressed as 4% of gross width. 4/96 puts that in the ball-park as a rule of thumb.