Here's something that might be of interest to everyone. A company by the name of Netgain Motors recently developed a prototype home retro hybrid kit called the EAS/EMIS (i.e. Electric Assist System-Engine/Motor Interface System.
The system incorporates a Warp 9 EV motor, which connects to the drive shaft of rear wheel drive automobile, which is then regulated by an on board computer. Upon acceleration or going up hills, the on board computer puts the gas engine in neutral and then seamlessly switches over to the EV motor to maximizes fuel economy. The increase in FE is said to between a range of 16-25%.