Net Gain has a very slick looking system of adding a hybrid electric system to an existing front engine, RWD, vehicle with an automatic transmission.
The electric motor boosts the gas motor for acceleration from a stop when you are getting about 2-8 mpg, as well as adding assist when needed.
I want to try it in my Expedition, and will keep you all posted on the results.
http://www.go-ev.com/EMIS.html
(BTW, i don't have anything to do with NetGain, but have ordered an Impulse 9 motor from them to convert my MR2, with a blown engine, to an EV.)
Tom