21ft² is fine, but unremarkable. You can see how it compares to
other cars. Your donor car, I believe, has 19-20ft² frontal area.
Your car will have to distinguish itself in its drag coefficient, then. The front of the car is sound (and, incidentally, not very important to overall drag). As to the rear, does it fit inside
this parabola? You can go slightly steeper than that template, but if your roofline gets too steep too fast, you'll develop a very high drag vortical wake behind the car.
If you can match the performance of a
specially modified GM Impact, you'll have a Cd=0.15, which is 60% better than the best production car ever sold. I'll throw that into the calculator for now.
Also assuming some very good LRR tires, 250g/kWh @ 65mph, summer gas, summer air, and a warmed up car, I figure 118mpg (1.99L/100km) at 65mph is achievable. These are all best-case numbers.
As you said, this is all just (very sound) theory and (well-informed) speculation, and the proof is in the prototype testing.