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Using very simple shapes to provide guidance for car modifications is invalid. Those simple shapes (whether they be The Template or an Ahmed body) are aerodynamics painted with the broadest possible brush. In the case of the Ahmed body, particularly, a lot has been learned since 1984.
It's rather like reading in an old book that for best power, the engine's air/fuel ratio should be 12.5:1, and best economy, 16:1. Of course, neither figure may actually be correct on a given engine: the best ratios can only be found through testing.
With the Ahmed body, and enlarging a diagram from an old book to get Cd figures in an attempt to guide car modification, it is doubly misleading.
First, the data applies only to an Ahmed body, a very simplified shape. Second, as recent research has shown, the Ahmed body results (and specifically drag changes from altering rear body angles) may in fact suggest the very opposite of what gives lowest drag!
And if the Ahmed body gives no actual guidance of use to car modifiers, why muddy the waters by mentioning it in the context of someone modifying their car?
Any time someone mentions simplified shapes and attempts to draw lessons from them for car modification, it's wise to be highly sceptical. Car aerodynamics is just not that easy.
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