Some consideration:
1) The 1961 Karmann-Ghia was Cd 0.39, and Af 18.793 sq-ft, CdA 7.32927-sq-ft ( 0.68091 m-sq ).
2) The test weight was 2,089-lbs.
30-mph = 56.3 mpg hwy
40-mph = 51.3 mpg
50-mph = 45.4 mpg
60-mph = 39.2 mpg
70-mph = 31.5 mpg
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The 1992 GM ULTRALITE, Cd 0.192, at 50-mph = 100 mpg.
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* The rolling force coefficient for the early Porsches was Cf 0.020832195
* Bridgestone Ecopias are Cf 0.0055
* The Beetle-based project could compensate for some of the mediocre 'Porsche 356' aerodynamics simply by incorporating the modern Low R-R tires.
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The BSFC of the air-cooled engine, from there on out, will determine the CdA you'd have to come up with in order to hit the 100-mpg target.
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Figure for a mechanical efficiency of 94% for VW's powertrain to get to your useful horsepower available at the traction interface of the rear tires.
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From this 'net' road power, subtract the horsepower absorbed by the tires, leaving what's left for aero.
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Knowing this horsepower, and your target road speed, run your drag power formula 'backwards' to solve for the target Road Load Force.
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Once you have this, allowing for the dynamic pressure/ air density/ velocity-squared, your target CdA will fall out of the math.
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Dividing the CdA by frontal area, yields your target Cd.
Then it's a matter of using 'known-knowns' from the aerodynamic bag-of-tricks reported in the last 102-years to get you to that 100-mpg.
