gas gage on 1996 Ford Mustang

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gas gage moves back alot. It will be at the half way mark and then i will look down a min later and it will go real close to the empty mark. And some times it will be near empty and i can go 45 miles to empty and other times i can only go 20 miles or so. what do you think is wrong?
(2) Answers
(2) Comments
either the sender unit is bad or you have a broken baffle in the tank allowing the fuel to slosh around way too much

One thing I tell everyone with this problem is to add a bottle of "Chevron Techron Concentrate Plus" into a full tank of fuel. This additive is the only one I know of that will clean the fuel level sensor in the tank (they can get dirty from fuel contaminants). So before you have any repairs performed, spend the $12.00 or so and try it, I've seen it fix several vehicles.
Good Luck