How about bike racks in the back? I wonder which placement is better for mileage due to aerodynamics: Top or back?
(but I realize that car shuttles requiring 4 40lb downhill mountain bikes isn't exactly your typical use case for a car! It's just an important part of what I personally need it to do...)
In my experience, bikes on the hitch rack hurt gas mileage less than the roof ones, but I haven't been super rigorous with the comparison...