I would like to bring some attention to Flux Hybrids. They can retrofit cars with an electric motor for huge efficiency gains (as reported on their website). They report an average of 100% increase in combined mpg. The cost of an HEV kit with installation is estimated to be about $10k. They have a PHEV kit that costs a little bit more.
I also like that they are pretty good about responding to my text messages.
https://www.fluxhybrids.com/