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This is what I found on the topic.

https://www.hemmings.com/magazine/hm...1/3183321.html

Quote:

The M1's trouble was weight, and surprisingly, aerodynamics. At 3,175 pounds, it was about equal to the Ferrari, but the Porsche weighed in at almost 700 pounds lighter. And despite its razor-edged profile, the M1's reinforced fiberglass body, mounted around an aluminum space frame, was alarmingly difficult to push through the wind, with a 0.40 drag coefficient. To put that into some perspective, a 1979 Mustang has a 0.49 drag coefficient. A 2005 Honda Odyssey has a 0.30.

I am surprised by the high numbers myself.

Many of the cars from the 1950's 1960's 1970's 1980's that look good Cd wise do not perform well Cd wise.

Things changed in the early 1990's, from what I can tell Audi was about four years ahead of everyone else for about a decade. Then everyone else caught up.