Problem with Scan Gauge MPG'S

OK got my scan gauge II been using it for a week now, but after getting gas and changing the amount of gas I actually pumped versus what the SG2 said my MPG"s have been way off. Is there a way to adjust them ? The instant MPG's are correct but the Current Fuel Economy and the tank MPG's are way off. I am 400 miles into this tank my MFD shows 61.2MPG and the SG2 tank MPG says 50 MPG . I was at 60.6mpg when I left work and it went up but when I got home my CFE showed 44.6mpg and it all started after the second fill-up ..any help would be greatly appreciated..thanks

 

I Thought I had my problem solved I went back to factory presets and gauge was good then I re-calibrated it (filled up at same station twice didn't make any changes until second time) but my current fuel economy is off again .. it was correct until second time filling up I changed the amount of gas I put in . I am going to wait until next fill up and see if it fixes it. The only thing I can think is the first time I got gas was in the cool morning this time it was 93 degrees maybe that is what did it..