I have had a scanguage 2 for a year or so now. I like many of the features it has like being able to monitor different guages and tracking mileage. I have recently bumped into a problem that has me quite frustrated. I'm having a bad fuel economy issue - truck is GUZZLING gas - and I've been trying to use the scanguage to help diagnose the problem. I checked the internet trying to find information on how the scanguage determines fuel economy so that I knew which sensors to explore in finding my problem. O2, MAF, etc. Finding nothing online, I called Scanguage's tech support line, explained what I was doing, and asked how they determine fuel economy so that I knew where to look. Their response was "That information is proprietary and we can't tell you."
I'm taking a guess here, but I believe the ScanGauge uses a combination of things:
The motor size you entered when you setup the ScanGauge
OBDII information on O2 sensor data
OBDII ECU data for injector pulse width
The correction factor you enter in when you fill the tank
At the minimum, these few items will allow the ScanGauge to estimate real-time fuel economy, but I'm not sure if it's using other information to help correct for any errors in the sensor data. I'd be interested to know what else you find.