I have a copy of the German magazine Motor Klassik that features a bunch of old cars tested in the Volkswagen wind tunnel. The data shows very clearly how optimistic pretty well all the Cd figures are that were quoted at the time of the cars' release. (Cw = Cd.)
Kamm K3 low drag research vehicle (Cd = 0.37):
BMW Wendler (Cd = 0.44):
Tatra 87 (Cd = 0.36):
All wonderful cars for their time, but obviously all with pretty bad Cd values in any modern context. I simply don't believe any old drag figures - instead, I look at the rated engine power and the top speed. For example, the Panhard Dynavia did 81 mph on 28hp, and if we accept these figures, it certainly had low drag.
