Doubling the gas mileage you get out of Toyota Prius would be nice! Well you can turn your little fuel efficient car in to a hard-core gas sipping, fuel saving beast with the addition of a Hymotion Plug-In kit for the Toyota Prius. The more your Prius can run in EV mode the better your gas mileage!
What does ten-thousand dollars mean to you? Most people spend that much on a set of dubs and a upgraded entertainment system for the Escalalde. Then there are those who throw 10 g’s away on all kinds of crap. Women probably spend that much on make-up and salon appointments in a year and men definitely spend that on beer and gambling in as much time. Well why not make the smart choice and opt for something that will save money and be less harmful to the environment.
The ability to run in EV Mode for a longer amount of time will no doubt increase the fuel economy of your Prius. The 
Hymotion L5 conversion kit turns your Prius essentially into a plug-in car. After shelling out the $10k which includes installation; you can charge the Hymotion battery pack which consists of A123 Lithium cells. It only takes 4.5 hours to fully charge the batteries from any household 110v socket. And Hymotion says you Prius can then get around 100 MPG for 30-40 miles. Being that the average commuter droves less than 40 miles in a day; this upgrade is perfect for those who are concerned about their MPG as well as the environment.
So you will have to drive about 200,000 miles to get your money back; but if Hymotion can get the price of this technology down than I guarantee this will sell like hot cakes!
Source: http://www.autobloggreen.com/2008/04/27/hymotion-launching-prius-plug-in-conversion-kits/