There was an interesting article in an issue of Circuit Cellar (now I'm really labeling myself here) about a mile/gallon display that someone built using the realtime information gained via the OBDII data port.

I forget the parameters he used to make the calculation but it was pretty interesting.

__________________
Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE

That 600 is a money pit.
Good luck.