I think you'll need to go all the way down for increase FE. I have the same problem and tried skirts that came down to the bulge and saw no effect on the FE. My driving is mixed, hardly ever get over 60 and most of the time it's around 30-40 MPH averaging about 30-35 MPH for a tank of gas.
The bottom of the tire is not moving through the air - ground speed is 0 so you don't have to cover it. The upper half of the tire is moving at twice the ground speed and needs to be covered.
Excellent point -- kinda like how tornadoes will destroy one house and the house next door is left untouched: there's a fast side as it moves and a slow side. I also see some camber issues (I think I'd run into the same problem). I'd experiment with the top half idea -- first though, jack up each side of the car and see if the top of the wheel pushes out at the top in any form during suspension operation. All hope is not lost.
1. Make one that doesn't go all the way down
2. Make one that bulges out a little bit and goes all the way down.
i think the full ones looks better. and even though the very bottom of the tire may not be moving relative to the ground, the wheel arch *is* moving through the air at full speed, so the further down you streamline it, the better.
mine bulge out a bit - see the photo in my DIY of the aluminum strip i attached to the back side of the skirt to get an idea of the shape i had to use.
you could cheat and stick a set of rear mudflaps on - that's what my skirts attach to (which reduces the amount of "bulge" required to clear the tire).