Just completed a 5 month 8,000 mile road trip. Saw lots of those side flaps all across the country. Apparently they are effective, or the truckers wouldn't use them.
I'm left wondering why they don't do more. Continuing the side flaps all the way to the rear of the trailer, covering the rear wheels, would seem to me to be very effective. A slight bulge would likely be needed for the wheel area. A simple hinged door, secured by Dzutz fastners would allow for tire changes.
In addition, I see no reason not to fair in the rear wheels of the truck itself. Surely that would result in an improvement. And, of course the hinged boat-tail offered here:
would offer considerable improvement, around 7-8 percent reduction in Cd, according to Hucho.
Apparently and industry is beginning to spring up around the need to have more aerodynamic long haul rigs. This site:
Offers several options. Their aerodynamically optimized trailer picture is very interesting.