Your Mileage May VaryThe plug-in hybrid (PHEV) version of Toyota's best-selling Prius hybrid, which is debuting in the U.S. next month and will be commercially available in 14 states this Spring (national rollout in 2013), was first supposed to be rated at around 87 MPG-equivalent. This alone would have been enough to put the plug-in Prius pretty high on the fuel-efficiency list, but after more testing, Toyota has revised its estimate and now says it is confident that the Prius PHEV will get 95 MPGe, which is 2 MPGe more than the Chevrolet Volt, which is rated at 93 MPGe. Be careful, though, as fuel-efficiency numbers for any plug-in vehicle can be very misleading...
Driving Habits Matter
Official MPGe numbers for plug-in hybrids are a lot less useful than MPG numbers for regular gasoline or diesel vehicles. The environmental impact of each marginal mile driven can vary a lot, between one extreme where the car is charged up with electricity from clean sources and is mostly driven within the electric range (the gas engine almost never turns on), and the other extreme where it is charged up from dirty electricity sources (like coal) is is almost always driven farther than the electric range.
So if you almost always drive short distances, the best plug-in hybrid for you is the one that has an electric range that is long enough to allow you to get where you're going without having the gas engine turn on. But if many of your trips are significantly longer than the electric range of the PHEV, it becomes more important to look at how efficient the vehicle is when it runs on gas (for example, the Volt has a longer electric range than the Prius PHEV, but it gets 37 MPG on gas while the Prius should get closer to 50 MPG).