There are 2 possibilities ( as Spock would say) for error when you use your fuel gauge to determine the colume in your tank:
1. Your fuel gauge could be wrong, either at the sender end or the gauge end. Typical of the old equipment.
2. Your fuel tank is not a perfect "cube." You can't depend on your fuel gauge to show the volume unless the tank has square sides that are all the same height and width, and the cross-section must also be the same all the way down from the top! Just look at a picture of the tank in your assembly manual. It's shaped like an inverted oil pan. That means the level goes down faster at the top and slower at the bottom. I can only get 75 miles on the first 1/2 tank and 200 on the second half??
the only sure way to use the fuel gauge to figure the volume of fuel in the tank is to fill it up when the gauge hits each mark. Better yet. find out how much to fill at every 1/10th tank. Every 1/100th tank. Still, the gauge isn't consistant...
Please don't make any more claims about fuel mileage using your fuel gauge as a volume gauge, unless you measured it scientifically.
Of course, it still is a good idea to know how far you can go when it gets down to 1/4, so you can make it to the next gas station
too sense.
-Spock out