I looked at the ScanGauge and
UltraGauge and settled on the UG. I set up screen 1 with avg and instantaneous MPG on top. I also monitor voltage, coolant temp, fuel remaining (very accurate), and mileage on oil change on this screen.
Mine calculates the mpg, and fuel used from the air ingested and has proven to be quite accurate. It took a few fillups to fine tune the calibration, but now when I fill up I'm always withing a half gallon of what it said I've used. Mine is set up so that it doesn't even look at the fuel level sensor.
I have the UG installed in my '05 Envoy.