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Wow ok guys some basic aero here.
an ideal teardrop produces ZERO lift. "HALF" a tear drop can produce lift.
next just because TWO forces are in play does not automatically mean the forces are directly connected.
The DRAG created by the Aero Top when it produces lift is NOT directly linked to the drag you lose by adding the top.
TWO SEPERATE FORCES acting on the total equation.
When you add an aero cover you are HURTING your mpg by adding TWO kinds of drag.
Parasitic Drag via the extra surface area and drag induced by the work of generating lift. These are seperate unto themselves.
NOW the cover mathmatically does NOT reduce drag at all.
it STOPS YOUR TRUCK FROM MAKING THE DRAG it was making before. a SEPERATE drag force completely unrelated to the lift drag and parasitic drag.
In rocketry we call this "base drag" the "suck" you get off the back side of anything flat.
SO why does the cover work?
because the values are UNRELATED to each other and NON EQUAL.
the drag you STOP the truck from creating by adding the cover is far far larger than the drag you ADD when you introduce the cover.
so you see and overall amount of drag that is LOWER.
one did not cause the other. this is the mistake your making. LIFT NEVER EVER reduces drag. Period. Lift by definition is "work" work by definition will induce drag.
but you can TRADE WORK from one piece to another piece for a net desirable goal.
Like using a pulley to lift something heavy. your exerting the SAME EXACT amount of energy (in fact MORE energy because of the DRAG you introduce with the pulley) than a dead lift.
but the pulley lets you spend less energy over a LARGER time period (so net same energy)
So don't think of a boat tail as reducing drag. Think of it as STOPING the car from MAKING the drag in the first place. same with the aero cover same with making some lift etc..
Lift is a consequence not an objective.
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