This is what I gathered from owning mine a few years:
It calculates MPG for that particular drive. Once you turn off the truck, IF it’s higher than the previous saved MPG, the gauge cluster will ask you if you want to save. If you open the door before you do that, it will disappear.
Drive truck, turn off truck, before opening any doors, notice if the prompt to save best MPG is available, select save with buttons on steering wheel.