GM calls the volt an extended range electric vehicle (EREV). It is an electric vehicle with a gasoline generator for extended range. The C-Max Energi is a plug-in hybrid electric vehicle (PHEV). It’s a hybrid drive with electric and gas drivetrains similar in design to the well known Toyota Prius. The electric drive must be more substantial for the Energi than the hybrid. I’m sure this is why the Energi gets slightly lower MPG than the hybrid when running in the hybrid mode. There is a lot of discussion about which approach is better, EREV or PHEV. Here’s two links (here and here) where General Motors claims the EREV is the better approach. What do you think?