The main problems you are going to have with trying to rig up a super efficient car is going to be longevity. If it were that easy to build something that got super good mileage, was safe, and lasted some car maker would already have cornered the market. And one did, just go buy a mid 90's VW with a 1.9TDI (diesel). I own an old one and a new one and they both get mid 40's low 50's for mileage and if you take care of them you will easily get 500k miles out of them. Plus if there is ever a fuel shortage diesel will be the last thing to run out (most people drive gassers) and diesel will be good for years (if not decades) whereas gas is pretty much useless within a year.