According to Toyota, its plug-in Prius hybrids are averaging 65 miles per gallon in real world testing. This is an improvement of 15 mpg over the recently-unveiled 2010 Prius. The secret to the big fuel mileage increase is a battery with the capacity to store much more energy than the unit in the standard Prius. This means a lithium ion technology and a much greater expense. So far, Toyota has not committed to a date when you'll be able to actually purchase a PHEV Prius for yourself, but it does plan to bring in another 150 test cars for testing.
Bill Reinert, Toyota's U.S. alternative-fuel vehicle manager, points out that this 65 mpg figure is from drivers who were instructed to pilot the test car the same as any other vehicle, so there are no hypermiling techniques required to achieve the mileage boost. Of course, the car must be plugged into an outlet to recharge its battery pack and fuel but mileage and range are still mostly determined by the driver's right foot. This is one reality that affects any car, including hybrids and fully electric machines.
[Source: Automotive News - sub. req'd]