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AHHHH.... Class A? Ok In order: Fuel tank is not calibrated to guage, in other words thats the way it is. Next: Bad sending unit. Next: Bad guage. I built a test unit for guages a few years ago with a wide range volume control salvaged from a old old radio. Testing the best way is to drop the tank and pull the sender hook it back up to the wiring and run the arm through a full sweep and have some one watch the guage. If the guage stops before the top of the sweep on the sender arm get a ohm meter and test the resistance for the sweep if you get even smooth change the guage is bad if you get choppy or a stop in change the sender is bad. Now For the wierd: I had a guagew in a bus once that quit working over 2/3 full and after changing the sender twice I found it had a pinched wire that only lost contact when the tank was fullish. Good Luck