There is a lot of skepticism on whether the Chevy Volt can obtain 230 mpg even as a hybrid. However, in one news broadcast on TV, someone referred to 230 mpg-equivalent.
I suspect what they did is take the milage in battery-only mode, and figure the cost in miles/dollar, then multiply by some value of dollars per gallon. The result is a miles per gallon if you spent the same money on a gas powered car. 230 mpg would be about right, since a GM press release says they are computing operating costs figuring 5 cents per kilowatt-hour.