Didn't the Volkhart-Sagitta V2 retest from 0.17 to 0.21 in a modern tunnel?
I gave it my best shot.
This could probably drop the Beetle from ~0.41 to ~0.30. The open bulkhead is at the stock rear bumper location. The solid bulkhead a reasonable truncation*.
The Tropfenwagen had a couple of other things going for it. The wheels and tires are exceedingly thin by modern standards (the chassis exceedingly thick), and the fenders/running boards are well in tune with modern time attack standards.
*That truncation on the left with a Coanda nozzle pumped by engine cooling air. Breers on the right.
