The stock original Insight has a Cd of 0.25 and the frontal area is 20.0 sq ft, so the CdA is 5.0 sq ft.
If you do a full aero drag treatment (bellypan, boattail, mirror delete, front wheel skirts, smooth flat wheels, grill block) and drop the Cd to 0.17, then the CdA would be 3.4 sq ft - which is a 32% improvement!
To do the same thing by deleting the roof etc., you would have to reduce the 20 sq ft frontal area down to 13.6 sq ft - which is very hard to do. The HUGE assumption is that the Cd would remain 0.25 - and I don't think it would. Doing this would completely rejigger the air flow.
What good is a car that doesn't have a roof?
The drivetrain is the single most important factor. Electric is the only drivetrain that will get a reasonably safe 2 seat car up over 200MPGe, and it will be be nearly silent and it will be *quick*. 85% plug-to-wheel beats ~35% any day.
Aerodynamics is the second most important factor. The Edison2 eVLC and
Allert Jacobs streamliner are case in point - they are significantly heavier but their low drag more than compensates. EV's have a big advantage here too - virtually no drag from the cooling system because they don't need one. Aerodynamic drag is a total loss.
Weight is important, but it is much more critical for an ICE than for an EV - because of the drivetrain efficiency of an ICE sucks, really. With an EV, you only lose 10% under acceleration; rather than 75-85%. Once the car is moving, that weight helps you coast - and an EV doesn't idle. And when you need to slow down, you get to use regenerative braking, to put some of the kinetic energy back in the battery. So, weight is both bad and good.
You can do über-low rolling resistance wheels; like the Energy Return Wheel; but you would have to address the aero drag of their design.
If you don't go with an electric drivetrain, you won't be able to have a car - again the
Allert Jacobs streamliner is case in point: it is a 125cc 231 pound slippery pumkin seed that carries just one person, and it gets "just" 214MPG. The Edison2 eVLC seats four tall adults and gets 245MPGe - and that could probably go over 300MPGe with an AC motor.