To add to the discussion:
1)
http://ecomodder.com/forum/showthrea...p-off-601.html
Relatively well done on-road testing shows that specific cap increased drag.
2)
http://ecomodder.com/forum/showthrea...ruck-1228.html
Obviously "traditional" cap styles vary a lot. Some increase frontal area, and have insufficient radii on the leading edges. Others are truck-specific and are nicely integrated into the OEM shape at the back of the cab.