That's a common question. There's no one product like the ScanGauge for pre-OBDII vehicles, but there are numerous possibilities if you're willing to do some work.
There's a link in my sig to a thread about a DIY fuel rate meter; that's very basic and doesn't report MPG, but in that thread I've got links to other threads with better ideas. I am particularly fond of the one that suggests using a factory MPG readout that came in a similar model of car.
This sig may return, some day.