Remember this nonsense?
In the summer of 2009, GM announced that its forthcoming Chevy Volt would get 230 miles per gallon. GM got to this figure by estimating the gasoline used by a theoretical driver who drove almost all of his miles powered by electricity available from the batteries, and only a few miles powered by the on-board gasoline engine.
The 230 figure was both meaningless and arbitrary. The calculation counts all the miles traveled but only part of the energy used; the estimated "MPG" could literally be anything one wanted it to be, and would approach infinity as the user drove ever more completely on the batteries.
The vehicle, we now know, gets 37 miles per gallon.
With the Volt on the showroom floors, meaningful MPG data and other useful information is available from its U.S. E.P.A. "mileage sticker:"