Scangauge calibration curve

I found that if I did a series of average mpg readings at calibrations settings from -17% to -24% while running at a constant at 55 or 60mph, the mpg values would rise and then fall. The highest point on the curve turned out to be the most accurate calibration value, -23.1%. This to me is counter-intuitive because when I took a series of readings at 40, 45, 50, 55, 60 and 65 mph the curve moved higher when I changed calibration from -22.6% to -24%, as expected by applying a constant factor.

Can anyone with inside information tell me the formula that the Scangauge uses to determine mpg or how the calibration can pass through a maximum?

Dick from FL
'06 3500 Winnebago "View"

Someone with more knowledge than I can correct me, (and perhaps I'm not understanding your question) but I don't think the calibration has anything to do with the average speed you are driving. My understanding is that the calibration is based purely on the amount of fuel that the guage calculates having been used in a tank vs. what it actually takes to fill it, and this is applied to the value obtained from the bus of fuel being consumed - with appropriate calculations of averages/calculus stuff to project 'miles to empty' etc.

But my Scanguage calibration bumps all over the place when I refill, from about 17% to the high 20s, so I'll be interested in the othe replies.

I've had my ScanGauge2 for 8 tankfulls (5700km).
I followed the instructions about topping the fuel off and adjusted the Unit accordingly.
Comparing the Unit's MPGallon and my own calculation with a calculator the first 3 tanks were out 20%,20% and 17%.
I recalibrated the SPEED function by 1 increment and now my in dash trip meter coincides with the MILEAGE ON TANK by 1%.
The last 4 tanks have a deviation of +/- 2%
Not to brag but I am getting over 21MPG in the city! (11.1L/100km) LOL

My speed adjustment at zero matches my GPS.
My sixth tank matched the gallons used exactly.
Calibration is just that. It isn't an adjustment you tweak all the time.