[sheepdog 44]

Wow, half you hp is used to overcome aero at 40mph! I think reducing the frontal area is the top priority. Theres only so much aerodynamics will accomplish on a car. You can get the drag coefficient down to around .16cd but not much less even on a completely optimized form. There's also only so much weight you can chop out of a conventional car. At that point you hit a wall and the only thing that will help is dramatically reducing the frontal area. If you can half the frontal area, you can half the CDA, and your numbers jump big time. Half hp to overcome aero is now 60mph.

I suggest stripping of all the bodywork and glass and sit in the center of the car. Take a picture from the front and and trace a line around whatever remains, being the wheels, suspension, engine and your body. That is your ideal frontal area, now just construct the cars body skin tightly to that outline, and then follow the streamlining template for it.

Thats the best aero treatment you can possibly get, you can just forget about interior space. Whats left is up to whatever propulsion system you use. And motorcycle tires could cut your rolling resistance at the expense of handling.