My new MPG was somewhat short-lived, because installing the giant hitch (Draw-Tite Class V 41906) hanging off the back both added 60lb to the van, and also totally ruined any chances of having nice clean airflow off the back. It took noticeably heavier throttle on the highway, and went back from 14.6mpg down to 14.0mpg. I'll figure out a way to smooth airflow around it, it'll just take some pondering to get it right!
Adding the rest of the side skirt behind the driver's rear wheel dropped my MPG yet again, but I think I know where I screwed up. I copied some of the geometry of the Volt, Prius and XL1, but utterly failed to keep the maximum departure angle in the 10-12 degree range. The exit taper is around 25 degrees, which is absolutely the worst possible choice! So while it's nice and smooth and looks pretty neat, it's not very effective. 14.0mpg dropped to around 13.7mpg with the rear wheel flare in place:
Outer foam all glued in place!
Some CAD (Cardboard-Aided-Design) of the behind-the-wheel aluminum cover, intended to protect the AC and heater lines for the rear and allow for easy access:
Inner structure with rivnuts and trimmed foam:
