This was common in New Zealand/Australia on Holdens (General Motors) and on Fords (late 1970's). If I remember correctly the guage was not calibrated and was marked somtheing like "Econo-guage". I believe that it was actualy a vacuum guage connected to the engine inlet manifold (or was that exhaust?).
Now my Ford Falcon (1997) displays this information on a digital display measured in litres per 100 kilometers. I don't know how they calculate it, but would not be at all surprised if it is just an estimate based on the inlet manifold vacuum.