I would like to say I love the idea, but I think 70 mpg is going to be a very hard goal to hit.
I generally get 55 mpg @ 60 mph with my Prius in summer. If I plug in all my data (2900 lbs, .01 Crr .26 Cd, 22.4 sq/ft) into the
aero & RR calculator and tweak my engine efficiency to .283, I can get the calculator to spit out 55 mpg @ 60 mph. Now, if we substitute the Prius c numbers in with 2500 lbs, the stock Cd of .29, and 22.1 sq/ft of frontal area (keeping the .283 engine efficiency and .01 Crr) we get just above 54 mpg @ 60 mph. If we bump that up to 65 mph we've now dropped down to just over 48 mpg. So, 48 mpg is roughly where you'll start. In order to get the 45% increase in mileage to get 70 mpg, you'll need to reduce the Cd down to roughly .167! That would require a TON of work to achieve. I'm not saying its not doable, but it is going to be a lot of work.