*Hucho's recommendation is to never do anything to add frontal area.
*The added frontal area adds drag arithmetically.
*If you add 10% frontal area,your drag increases 10%,all else being equal.
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*Don Bur,in the UK has been able to add frontal area to create roof camber on his '18-wheelers' ,and realize a net drag reduction.
*If you're absolutely committed to the roof addition,you should study their work closely.
http://www.fdcholdings.co.uk/wp-cont...rop-tunnel.jpg
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*Having roof camber,without side camber is a setup for creation of vortices if not integrated perfectly.
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Also,you owe your self a look at 'NOSECONE' fairings.These are undisputedly the best solution for the front of the box.Truck salvage yards might provide you with a used one which would fit the truck.I worked for a local moving company and we added a nosecone to a 35-ft, Ford chassis,gasoline-powered, moving van and realized 8-mpg with it.Something otherwise impossible without it.
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I've been working on commercial vehicle stuff during the heat of the day and will try and post some truck aero stuff Saturday (tomorrow).
If you haven't seen the NASA truck aero link yet,check it out.It's way out ahead of nothing.
