[hat_man]

I think I'm understanding it better now.

So the most efficient "model" would be the AST viewed in 3-D. Like a teardrop. Not practical for a vehicle so a near half teardrop with the road being the mid-plane (?) of the teardrop.

So another question.....in my mind I'm seeing a half teardrop with lines representing the (laminar?) flow along it's surface. As the pressure travels rearwards along the teardrop (and comes back together) does it's cumulative effect "squeeze" the tail of the teardrop causing theoretical forward movement? Like pinching a watermelon seed between your thumb and forefinger and shooting it at your wife? So if you rob some of the pressure through the middle (6" tube) you decrease the amount of pressure along the sides of the teardrop and lessen the "squeeze"? Not only would it decrease the pressure moving down the sides but could create drag along the insides of the tube because of the friction?

That would be why a boat tail works so well? It helps direct the flow along the sides into the low pressure void at the rear of the vehicle? I can see now how an aero cap and a boat tail would be beneficial for a pickup truck.