I have always wondered what an efficient non-hybrid car could manage with all the ancillary efficiency features that comes on the hybrids, such as ultra-light alloy wheels, low rolling resistance tires, aerodynamic body modifications, and instant mileage display, without actually being a hybrid . Those types of changes would easily be doable as well (The mileage display can be done with an aftermarket OBD-code reader and the right software), and should probably be the first things done on such an experiment.

Making a true homemade hybrid though is certainly possible. I remember reading an article about this vehicle made by high-school students in Pennsylvania. They used a 2-seater lightweight kit-car with a Jetta TurboDiesel engine powering the rear-wheels, and a 200hp electric motor powering the front wheels. Splittng the electric and engine-powered driven wheels would probably be the easiest way to do a home-made hybrid. To modify a stock car, the best way would probably be to start with a part-time 4wd car (so that it has axles for both sets of wheels), but replace the transfer case with an electric motor dedicated to the rear wheels, and let the engine be dedicated to the front wheels.