Some comments:

1) The key issue for electric-only propulsion is electric energy storage. Right now the affordable, mass-producible electric batteries are both heavy and not energy-dense compared to a tank of hydrocarbon fuel.

2) Electric propulsion is the most practicable way of recovering energy via braking. (This can also be done via pneumatic propulsion). The Prius has manufactured this successfully, but it can be made more energy efficient by further battery improvements or using energy storage via supercapacitors. There has been reporting of a major improvement in the ability of Lithium Ion batteries to charge much more quickly which could advance this part of the field and make supercapacitors unnecessary.

3) Why not hook up a small diesel motor to an electric generator, this should result in greater savings since a smaller horsepower motor running at optimal rpms should be more efficient than the current gasoline engines.