[aerohead]

Originally Posted by drmiller100

40 to 50 percent is the magic number.
for posterity, the BEST set of articles I have ever seen for those interested in this stuff.

Coventry University - The Aerodynamics Challenge

If someone doesn't beat me to it,I'll get Kamm's work posted.You'll see that his tail ended at 50% frontal area,as at this 'length',the lack of clearance for the SAE 10-degree departure would have knocked a longer tail off the car.
Fachsenfeld solved this challenge with the inflatable section to complete the rest of the tail,only on the open highway.