Toyota’s current plug-ins are:

...being used for auto show displays, brief demonstrations and university studies. They are very early “converted” prototypes using double nickel-metal hydride batteries. Although mileage data has been collected during demonstration events, it varies too widely to offer any indication of what to expect consistently.

Most importantly, these vehicles are not representative of the lithium-ion battery-powered next-generation Prius PHVs that will be delivered to lease-fleet customers later this year.

When asked about the mpg performance of PHVs—either lithium or nickel battery powered—we have consistently answered that we believe that PHV mileage will vary dramatically, depending on driving style and driving conditions...even more dramatically than with a conventional gas-electric hybrid. This is due to the added weight of a large battery that, once depleted by pure-electric drive, will contribute nothing until it is plugged in and re-charged.