I can't come up with a figure, either, but I did find the 1921 Rumpler Teardrop-Car's 0.28 Cd in "
World History of the Automobile", which might be a book worth reading.
What a beautiful car. Its rear fenders illustrate how to add illuminated width to your streamlined car without adding much drag.
14x5.5"? You can fit the 12lb LRR Insight tires on there.