Holy crap Vekke, you take this stuff seriously
This was just a mock-up model but you make me want to do a precise one now. It'll take a couple of hours - thankfully I have tomorrow off work so I'll produce something then.
Only thing is, I can run the simulation and tell you the air velocity, but I'm not sure how that will translate into fuel savings. I've got no education on aerodynamics as yet, just a bit of modelling skill and plenty of enthusiasm.