I am curious as to how? ? or when

I have had my sending unit out and was messing with range from full to empty and it never did anyhing but go to the low fuel indicator to full and all around really slow by the way it is a delayad input so it doesn't bounce on the needle

 

Try cleaning your positive terminals on both batteries. Advanced auto sells terminal cleaner that neutralizes corrosion on battery terminals. It's red when sprayed and turns yellow when activated on corrosion. 95% of the time fuel gauge will read low fuel or show gas all the way full even though you know it isn't. Clean the positive teminals and check. I can almost guarantee that will solve your problem with fuel gauge acting up out of the blue.