[MetroMPG]

The trouble as I see it is there may not be any attached flow on the top of the roof because of the flat, vertical windshield blasting the air into turbulence upwards & to the sides. If that's the case, then a rear taper may not help much.

I'm not certain, though. I bet Phil could chime in with some sage advice. I'd say it's worth starting a new thread in the aero forum (plus that's the only forum Phil reads). I'm betting you guys won't be the last Jeepers to mull over this idea.