So sorry Frank Lee. I have not 'blown off' anybody. I just found your answer too brief and uninformative.
I have a library of over 20 books on aerodynamics and aircraft design from when I was an active aircraft mechanic, EAA and experimental aircraft builder. I have a further dozen books on marine hull design and flow dynamics along with analysis programs like Delftship, Michlet Water Resistance Calculator, ArchimedesMB Flowdesign. None of them give specific answers to my questions. I have books on racing suspension, cooling and general body aerodynamics that have not answered my question.
Plus: It was suggested (by you and several other people on multiple threads) that instead of driving a truck/ other un-aerodynamic / high HP vehicle to just buy an economy car. My answer to that is:
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Why Do I Drive A Gas Guzzling Truck
All the details here to prevent those unusable helpful suggestions -
Driving a pickup truck is a necessity, not an option for some of us. I travel to a job site, unload and then live locally for a while [2 weeks to a couple months]. While were ever I am, I am driving solo with an empty bed. The load I carry occasionally prevents me from using a flat hard tonneau or a shell. I tried renting a trailer but sometimes I have to leave one job site to travel to the next on hours notice making rental impossible (told at 5PM to go pack and be at new job site 2/3/4/5 hundred miles away tomorrow or day after but really just barely make-able; now find a rental place open). I even tried a car/trailer combo for a year but hotels and motels parking policies make it difficult to just impossible. Or it gets broken into and vandalized, three times in a year. Or, finally, the tire I had removed and locked inside did not make a difference, I got back from the job one day and it was gone. Back to the truck. So now you understand why I drive a pickup truck and can not just “buy a beater for the commute” like I see suggested so many times on the threads.
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I know that just smoothing out the airflow is not always the best thing. I know that sometimes the after body drag / turbulence can be so high as to negate or even worsen mileage. IE: the tailgate being up is better on trucks is counter-intuitive and takes wind tunnel or CFD to 'see' it. And various streamlined cars have had to have drag inducing spoilers added to restore performance and handling lost to poor air flow AFTER the car [ex; the TT].
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Seriously that little lip isn't worth all this obsessing.
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Recently here on the forum there was even a video link here about how Ford trailing edge tweaks made such a difference in mileage. You may have seen it but did you SEE the point? Little lips sometimes DO make a difference.
And Big Dave, I like where you are going with your truck. I am following a similar path. However, my first duct tape, lath n plastic tarp tuft testing showed there is HUGE turbulence on the side corners at the tail lights and on the tailgate. The tufts did not just lie in different directions, they move back and forth and swirl around. That means there is a LOT of drag and because the basic shape I formed has the right angles (less than 12 degrees over the top and complying with the template and tapering on the sides at 7 degrees)
I do not think there is any significant change to be made to the cover shape. That leaves trailing edge spoilers and vortex generators.
On another thread a reference to vortex generators on the trailing edge generated disparaging remarks that what good would they do because they worked to keep the slipstream attached to the following surface and if there was no surface there would be nothing to work on. And that is just not true. Search for yourself, lots of truck testing has positively shown that vortex generators on the trailing edge break up the large drag inducing vacuum vortex into multiple smaller ones that cause less drag. Drag, meaning mostly after-body drag is a major reason for lower mileage!!!
So there is a problem that I want to fix as best as possible. I am looking for that little tweak that I am sure is there. I might have to build a sub-kammback on the tailgate itself but that is why I am asking if anyone else knows about this. If I can find a "Ford Fix" it would be easier.
PS: I have installed an electric fan, blocked the grill and have consumed 3 sheets of coroplast sheathing the underside, including a difuser from the axle to the bumper. I am still looking for more information on correctly shaped tire dams.