With plug-in hybrids, in particular, the phrase "your mileage may vary" applies... a lot. That's because much of it depends on how far you drive between recharging the battery. These results are based on the EPA's assumptions about how many miles a typical driver would run their car on gasoline each year.
The Prius Plug-in travels only a short way on electric power, especially in extreme temperatures, but it's enough to boost efficiency, especially if you can recharge a lot.
Honda's new all-electric plug-in is fun to drive, has good range and doesn't disappoint with space.