I have a ScanGageII add on scanner and it has displayed 9,999MPG on deceleration, also 1.1MPG on acceleration with a heavy right foot applied.
Have tucked with it in 5 feet of a 18 wheeler at 55MPH and gotten readings of 45MPG.
What you maybe seeing with the 23MPG is a steady cruse reading at speed.
The ScanGageII has a way one can check day to day the average from today and the day before, the best way is fill up to fill up using the same pump ant the same station and using the miles from the dash to figure the MPG number.
The ScanGageII has the MPG that is very active and one can see how the heavy right foot does effect to over all MPG, it has help me get almost 24MPG highway when traveling and better than 17MPG city streets.
My trucks sweet spot is about 53-54MPH, flat, not drafting 35-36MPG.