Yeah, I was wondering about the accuracy of the scan gauge, and how it stacks up to long term tracking of fuel use.You should build a spreadsheet. Enter your trip meter and have it correct for the tires. Use that mileage to hand calculate fuel. I bet that scan gauge is off. You can use the spreadsheet also to compare each tank to the scan gauge to give you a running tally of it's error.
I've done this in the past with oem dash meters. It's surprising.
My current work hack tells me fuel usage. I've run out out of fuel when it said I have 30+km of range left.
Apparently, that's 30 km of downhill only range