Here is the rear of the Cummins tractor. The back end of this vehicle looks really cool.
But notice how the side skirts roll around to the rear fascia with those large radii. This is obviously an aero benefit on a non-combination vehicle to fill in the rear vacuum. I would not want to do this on a combination vehicle as I would not want that air on the side of the tractor skirts diverted to underneath of the trailer.
My new truck has 90 degree angles at the trailing edge of the tractor skirts to the rear fascia area to maintain the side air flow of the tractor skirts, all the way down the trailer skirts and the boat-tail. Here is a teaser image.
By the way, we should have some "teaser" images of the new truck that I can post this month. And we have a totally, way cool, Augmented Reality (AR) program that we are working on for the new truck. That AR technology is so amazing. No headset/visor is required, as it is view able on a tablet. I have seen the first version and it is amazing, but we gotta tweak it a little more. I'm hoping to post the AR video sometime later this year.