This is a paper from 1992 on the VX
I was wrong, the CD of the VX was 0.31. So aeromoding would do a lot of good, huh?
I was also wrong on the highway fuel economy. The VX was good for 55 mpg!
According to this paper, the VW had a weight reduction of 80 lbs., which was good for a 2.5% improvement in fuel economy. Going from 0.32 to 0.31 on the CD was good for 1.5%.
I think that you could get a pretty significant weight reduction with the carbon fiber, etc.. 100 lbs. would be easy. 200 lbs. might be possible. Would that be good for a 6.25% improvement in FE?
Could you get down to 0.25 on the CD by lowering it, an air dam, mirror delete, and a spoiler? Would that be good for another 7.5% improvement?
Engine mods maybe get you another couple of %. Let's say 2%.
That's a roughly 15% improvement. You'd need a 81% improvement.
Can that gulf be bridged by the electronics mods (start-stop, engine cut off on decel, alternator cut off on accel)? And driving techniques? And tuning?