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Hi Metro and Trebuchet,
You can see what momentum effect of the air in the Jeep picture both after the hood and straight up from the windscreen.
See that one slip streem that goes up, then down, then way up again. That is the effect.
Remember the width of the car applies. So even a 1 inch decrease in flow height will be advantageous. Kinda analagous to lowering the car by one inch.
In the Jeep picture it looks like there is more like 4 inches of cross-section reduction that could be acomplished, but not with VG's.
Looking again at the Mercedes picture. Its there too. Look at the streamline that comes over the hood the closest and follow it. It disappears into the next higher streamline right at the windshield roof transistion, and both streamlines disappear and reappear further back. So, maybe 2 vertical inches of momentum effect, and then a turbulent section 1/3 the way back on the roof. It looks like there is much more tubulence in the boundary layer on the roof from there back, which just degenerates into flow seperation at the C pillar.
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