John,here's a link to the Trailer Tail's website
ATDynamics – Fuel Efficiency Aerodynamics Technology for Tractor Trailers saving over 10% with TrailerTail and Side Skirts
Here is a drag study conducted by Kamm/Fachsenfeld
Here's another relationship
The underbody and tire drag will contaminate the drag relationship,but there's a definite trend as the wake area is reduced.
The air is trying to 'tell' us what it wants
Brett's fiberglass magic is certainly pointing the F-150 down the right path.And the extra booty should only add to the fun!