>> Maybe not digital, but what about the analog MPG meters that BMW's have had
>> just below the speedometer for as long as can remember.  I'm pretty sure
>> these existed before FI replaced carburation on BMW's, maybe not.  Anyway,
>> these meters respond very quickly to driving conditions.  Any idea if these
>> used a flow gauge?

>These, as do most "economy guages" use a pressure sensor on the
>vacuum line of the engine. The harder the engine work the higher the
>vacuum. Very imperic.

Do you mean "less pressure" when you say "higher vacuum"?
If so, you are mistaken.

If the engine works harder you get MORE pressure in the intake
manifold.

Just think of it, when you step on the loud pedal you open the air
intake valve to allow more air into the engine. More air -> Higher pressure. 

On a regular suction engine this pressure will be approx 1 bar, equal to
the pressure of the outside air.

The above is not true when it comes to turbo or compressor charged engines.

-Oyvind