IIRC, there are one or two companies that make a retro-fit kit for the prius that turns it into a phev (plug-in hybrid electric vehicle). The AER (all electric range) is reportedly 30 miles or so, in empirical testing, but limiting speed to 34 miles per hour. Here is the link for those interested, no affiliation:

http://www.edrivesystems.com/faq.html

What bugs me about some of these folks doing the R&D is that they report an mpg increase as up to 500 mpg as possible. They don't take into account the cost of electricity to charge the battery. They could just as easily claim one billion mpg by their way of thinking.