Sorry. I've studied aerodynamics since 1981 as an 11 year old, and haven't seen it published
It varied so much between car that its not possible to define it precisely, Straight Six.
I know Ford only used scale model tests at the Lockhead wind tunnel in Georgia, and someone may know the answer for full scale models within Ford. Generally, Ford Policy since 1982 is NOT to quote cd figures on older than 1977 cars. This is due to some major issues with the possition of the car when tested, and before the 1977 Ford Fairmont, a wind tunnel testing wasn't very strictly enforced.
Alteration of
a) tire section size and style
b) ride height,
c) the front to rear ride height relationship
d) door seal firmness
e) the wing mirrors, wipers, ariel and spoiler configurations
makes a huge difference.
Ford was subjected to SAE queries and independent testing at MIRA in England over the 0.385 drag factor claimed for there 1981 Escorts . For example, a European 1981 Ford Escort varied 13% depending on the a, b, c, d and e.
Since the 1983 Thunderbird days, Ford Dearborn has supplied a lot of its older information to technical institutions all over the world by SAE contract. People who break ranks, and divulge the information have had there technical support being withdrawn by Ford.
For example, a Boss 302 or 429 was way more slippery than a 69 Fastback, but how much is anyones guess. The Holmon Moody race cars were far better than that because the whole car was radically channelled at the front.
For any fastback, I'd use a default cd value of 0.45, and frontal area between 1.95 for the 64.5 to 2.10 meters square for the 69/70 . To get that in good old Imperial sq feet, multiply by 10.76
Any time you have a Grande or notchback, that drag factor gets a lot worse, say 0.48. Any time you add fat tires, it raises the drag factor, any time you lower the car, it lowers drag.
Hope that helps some.
