[RedDevil]

Originally Posted by NeilBlanchard

(...) I tried smoothing over the trough along the windshield at the 'A' pillar and it did absolutely nothing.

I've been looking over that Mercedes with its reputed Cw of 0.23 to look for details that are different, as from a distance it looks like nothing special is going on.
On of those details was that the A pillars have a protruding edge, as if the glass has sunk half an inch down.
Then there's the 2015 Jazz, which (as a hybrid) claims to get 80+ mpg. It has that same feature...

So (at least) 2 cars that get better FE than expected have protruding A pillars, or 'deepened' front windows.
What then makes that so good?
You'd expect that it would be good to make the air roll smoothly over the A pillar. These designs, and the trough you covered, all make it less smooth, but they work.

In the Mercedes smoke test you can see the air still rolls over the A pillar, apparently the edge cannot prevent it completely. It makes a sharp radius and gets turbulent. It does not look particularly good, something you want to avoid.
The flow over the roof is very smooth indeed.
I think anything that can lead more air over the roof instead of curling over the A pillar would lower air resistance. Might even add a small ridge to the front of my A pillars one day...

When your mod doesn't do what you expected, it may just be pointing out another direction to follow.