Dodge Charger Daytona backlight drag
 

Here is a pictorial drag table based upon data presented in the SAE Paper identified on the image header.
I deleted the Daytona's rear wing/vertical stabilizer and adjusted the Cds accordingly.
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The backlight research was actually conducted on the 1969 'RACE' Charger,which was carried onto the follow-on Daytona for the 1970 NASCAR season.
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*The center image reflects the actual homologation production Dodge Charger Daytona/Plymouth Roadrunner 'Superbird,utilizing a 22-degree backlight angle.
*The 18-degree backlight had the lowest drag but did not produce the negative lift of the 22-degree 'compromise' configuration,do to the C-Pillar architecture specific to the Charger/Roadrunner.
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*In all configurations,the roof-to-boot angle remains approx. 15.5-degrees.
*The drag varies with downwash/sidewash interaction particular to each configuration of the notchback.
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The Daytona 'package' produced a 16% drag reduction over the 1969 'RACE' Charger,and reduced the Road Horsepower at 190-200-mph,by 85-hp,giving about a 5-mpg 'lap' speed increase.On a straight course the car was capable of speeds in excess of 220-mph.
http://i1271.photobucket.com/albums/...n12-21-133.jpg

NASA Langley aero guy told me 30 years ago to keep the angle <~8 degrees to keep flow attached. That was on a body of revolution, though, but comparable concept.

8-degrees
 

*W.A.Mair's boat tail research revealed 22-degrees as the maximum aft-body contour slope which would maintain attached flow on a streamline body of revolution.
*Hucho tells us that Rolf Buchheim et al. recommended no more than 23-degrees.
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*Buchheim et al.also stipulated that for simple aft-body angles,that the angle was completely dependent upon the precentage of aft-body compared to total length of the car,for fastback cars.
*Buchheim et al. also stipulates that this slope angle will change if the the roofline and C-pillars have curvature leading into the slope.
*For notchback cars it's a whole different ball game.
*And as for 'simple' angles,the only applications for such things today would be as with the "Trailer Tail" folding boat tail,
'n maybe JEEP Wranglers,HUMMER,etc..Everything else has critical curvature and camber.

"The Daytona 'package' produced a 16% drag reduction over the 1969 'RACE' Charger,and reduced the Road Horsepower at 190-200-mph,by 85-hp,giving about a 5-mpg 'lap' speed increase.On a straight course the car was capable of speeds in excess of 220-mph"

WOW!

220
 

Not too many years ago,a fella took a Dodge, Nextel Cup,NASCAR racer out to Bonneville.It was inferred that the car had a regulation restrictor-plate engine.
The car exceeded 240-mph.
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Somewhere,Alex Tremulis commented that his Subaru X-100 Gyronaut got 100-mpg on the straits at Ontario Motor Speedway,and only 80-mpg in the curves.It's the only MPG reference I've seen since 1974 regarding the impact of 'Curve-resistance'.

btw, where is that 'plate'. I think there is one in the Sportage and I want to take it out!!!!

Yes, the winged warriors were quite the project. The whole push behind Dodge creating this was speed...5mph over the "normal" Charger. They either had to squeeze almost one hundred hp out of the mighty 426 HEMI (which would take lots of time and effort) or reduce drag... The wing was necessary to keep the traction on the rear wheels at speed. :D
The Plymouth Superbird and Dodge Charger Daytona, supercars
More info: The record-setting 200 mph Dodge Charger Daytona Development: Dodge determined to win at NASCAR; development of the Charger Daytona

What amazes me is that this many years down the road, few vehicles can boast a better Cd.

And by the way, the 1969 426 HEMI would make the advertised 425hp with one spark plug wire pulled. It was a strong engine...
*edit* The picture Aerohead posted also helps explain why Plymouth's aero department really wanted to change the rear window angle on the Superbird.

Found a wind tunnel video of a 71 Superbird that was destined for Richard Petty... http://www.youtube.com/watch?v=zl7jk85D2sE

You must have some fancy computer software or a really good slide rule.;)

SLIDERULE = 3-digit accuracy

...so, if you're only interested in X.XX values, it gets the job done fine (wink,wink)!

And, "YES" I still have my sliderules:

1) metal yellow Pickett
2) plastic Post,
3) bamboo/plastic Pickett
4) 3" dia. circular Scientific Instruments Co.