I believe that the Volt can teach us a lot about what to expect from the Bolt when it comes out. The Volt has an electric engine that has a backup gasoline engine to assuage people's range anxieties. As such, the Volt has two separate ratings, 106 mpg-e for the electric motors and 42 mpg for the engine/generator.
The guys at TFL bet that the Volt would actually get 75 mpg or better when tested in the real world. Check out the video below to see if they're right.