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Update:
I taped four rows of tufts to the bed cover. The leading row (nearest cab) was pulled forward and down strongly while the remaining rows were pulled backwards but wagged side to side.
I waited for a very windy day (30 ish mph), parked my truck into the wind, and threw handfuls of sawdust on the windshield and watched it flow it over the cab. After flowing over the wing the sawdust took a sharp turn downward and a lot hit the leading edge of my bed cover, with the bulk of it hitting the bed cover and flowing off the back.
I then changed the angle of my wing to about 7 degrees. This equates to about 11" above the tailgate. The wind and my free time haven't coincided since, so I sort of repeated the sawdust experiment by having my wife release a handful of sawdust as we were driving down the road. The sawdust now missed the bed cover and passed over the tailgate a few inches above it.
The tufts now show the leading row of tufts to pull weakly forward and slightly down. The second row flops around but mostly rearward, and the last two rows are pulled back fairly strongly.
So what did this do for mileage? Don't know. I had to replace the EGR feedback sensor and the spring weather has been erratic so I can't be sure. My next step is to build adjustable supports, find a long hill, and do some coasting tests at different angles. May also build a rig to release sawdust/flour/cornmeal ahead of my windshield so I can see airflow as I drive down the road.
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