I've had some problems with the MPG readings on my ScanGuage II on a 1999 Saturn SW. When I first installed the SG on my "new" (new to me, 94k) wagon, I found the MPG readings way too low (in the 20's, while I knew from fill ups I was getting high 30's). In a thousand or so miles, I played with it a bit, trying different things. Then suddenly it started working correctly. Unfortunately, I had been toying with it without really keeping track of what I did each time, so I don't know what I did last. But either way, I don't think there's really anything in the SG to set, other than the tank size which won't effect MPG.
So, everything was fine for a 6,000 miles or so, then I had to take off the SG, and suddenly I'm back to the problem of low MPG readings (again, I have checked this against actual miles and fill ups).
Well tank size and engine size. The little blue manual that comes with it also shows how to alter the readings a bit to better match what you are getting with your personal calculations to adjust for slightly variable tire pressure or tire size variations. You're "new" car may have had the original tires or rims changed out.
you can got back to default setting and then you just start over. from main screen. from home pusy more, more , more then it should ask if you want to use defaults. then it'll be like when you first installed it. you'll have to set for tank size, etc.