Math Help - miles per gallon

I think that the rev counter on cars could be replaced by a counter that shows the current fuel efficiency.
This would be simple to do but what I wonder would be the formula for calculating it if you couldn't just measure the fuel intake against the speed.
What formula would apply then?

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.

It certainy should not be difficult to have a meter on the fuel feed that measures, say, L litres per hour. With the speedometer that measures K km per hour, km per litre would just K/L.