I think I may have misread your posts. My apologies.
It's intended as an estimate of what we can reasonably expect if we spend a weekend in the garage with some coroplast and a set of xacto knives.
There's a lot that can be done.
*tapered rear roof spoiler for 6:1 fineness ratio
*rear wheel skirts
*front air dam
*removal of exterior mirrors, replace with cameras
*lowered ride height to 3"
*lens covers for headlights
*smooth wheel covers or smooth rims
*shaved door handles
*removal of all exterior projections such as radio antenna, trim pieces
*skinnier tires(reduces frontal area)
*decreased gaps in door, trunk, ect.
Some of these are more expensive than others. Most can all be implemented for cheap with things from a hardware store.
That Calibra above already has some of these things direct from the factory. Thus the drag reductions observed are far less dramatic than from any car you might buy in the U.S. Most cars sold in the U.S. still ignore the rudimentary bellypan, that is usually standard in other countries.
Phil Knox has a Honda CRX. From the factory, it had a .29 drag coefficient. He got that Cd down to like a .19 or so.
Guess what that did to fuel economy? He had no other mods except for aero.
Got 90 mpg highway.
Now your home hobbyist won't be able to do all of them, but some that seem hard, like the rear boattail, can still be done with coroplast.
The maxmpg group on Yahoo.com has some members with Honda Civics that get ~70 mpg highway. They have rear boattails and other mods made from coroplast. Normally these cars would be hard pressed to get 50 mpg highway.
The gains to be made from aero alone are way more than 5-10%! More like around 20-30%, with the basic stuff you can do in your garage. More with the more expensive(but less dramatic) mods added.
Another 20% or so can be gained frm driving technique.
Another 25% or so can be gained from LRR tires, low friction wheel bearings, machined brakes so they don't drag, synthetic transmission oil, and other things.
Then there are small but significant gains to be made from engine modifications or swaps.
I believe krousdb did most of these things, and doubled the fuel economy of his Del Sol. Still didn't tap aero for all it was worth though, and I'd probably guess that driving style accounted for about 50% of his gains(codfishing and other driving techniques).
There's a guy here with a 60 mpg Civic. He attributes most of the gains from aeromods, I believe.
Interesting article. But you must keep in mind the Calibra is already relatively aero to begin with, and when conducting these tests, the overall body profile of the car remained unaltered. A tapered rear end among other things would greatly reduce drag yet more.
TC, note that tapering the rear end assumes a world without crosswinds. And it might well be valid if you lived in such an area. Dark areas are windy in the map below.
Tapering the sides of a vehicle (not the top and bottom, tapering those is a GOOD thing), will adversely affect the drag coefficient in a crosswind. And especially for an EV, that's an especially valid point. It's no good being able to go 300 miles if the weather is right and 150 miles in windy conditions.
(If you understand what I'm talking about ignore me, btw.)
Consider how well a shark would swim if it was forced to swim perpendicular to its usual motion. It wouldn't. It wouldn't swim very well, even if it had to swim at a 30 degree angle of attack.
Now consider a stingray. It can pretty much go where it wants to, independent of current.
And now back to cars. The primary reason for aerodynamic modification is load under highway. We can virtually ignore low speed aerodynamics. And this changes things. Unlike the ray, we aren't concerned about drifting above a specific area at slow speed. We are only concerned about minimizing drag at speed, in a crosswind or otherwise. Thus we are only concerned about Cd under a fixed range of angles of attack, not all angles of attack.