I put full solar panels (9 units) across the entire top of my 88 suburban, starting at about driver head position, going up over the factory roofrack bars, continuing across roof to about 3 feet from the back of the suburban at a height of about 3 inches off the roof itself, then last panels droop down to perfectly meet the factory "angle roof corner mini spoiler" on the back of those suburbans of that era, at an altitude of about 1.5 inches. fairly smooths out the roof, and that slightly humpback shape across the entire roof length now gave me a surprise of about 1 mpg increase, which from about 1 month ago, has been consistent. Really surprised me, was not expecting anything like that, and as I only used to get about 10mpg on average, was really happy to get a 10% increase in my mileage out of a gallon of gas..... Will try to take a picture of the setup tomorrow and post.
I really do think that possibly the turbulence started by the roof rack, somehow creates a bigger vortex vacuum on the back side of the suburban, and eliminating this helps a lot. If you put a 6 foot long closing cone at the back of the suburban (like those aerodynamic cars did from the 30's), who knows, but I'll bet you get huge mileage uptick..