Anyway, the roof. Angle up, flat, angle down in better than nothing. Or, you can go flat and then bolt something round down up there. I hear you about flat angles being easier to construct. That's why houses, campers, etc, have right angles and flats - ease of construction. If we built things round it would take longer.
I don't know how much better fuel economy you can expect. I think 25 mpg is dreaming. 20 mpg? who knows. Fun stuff nonetheless. You can start brewing alcohol for fuel if you get fed up enough.
From what I know about aerodynamics the vortex generators you are seeing on the rear of these vehicles are completely useless. It is no surprise they are increasing drag. The purpose of a vortex generator is to keep the flow from separating. In otherwords keeping the flow laminar over the surface. If you want them to do anything you should attach them at the front of the vehicle.
Two of my mechanical engineering peers did a project this past semester and found the biggest improvement you can make to gas mileage is to follow other vehicles and effectively draft off of them. If I recall correctly they found it effective to follow vehicles even out to a range of 200ft.
I had a jeep when I lived in england. With my 32" tires on stock gears,I would have to keep the throttle pinned in order to keep up with traffic on the highway.. unless I got behind a lorry. I would be cruising about 15 feet off the back of these big huge trucks, barely touching the throttle.. huge improvement... albeit fairly unsafe:Wow1:
That's probably why I frequently have someone ridding my bumper for many miles on the highway. Think maybe I'll start looking for truck to draft behind.
It is like F1 racing, any 3 cars in a pack on the track are faster than any single car due to aerodynamics.