1996 Fuel gauge and miles to empty problems

I just bought a 1996 Taurus GL V6. The gas gauge always reads full and below the gauge the miles to empty (I think) always reads 0. The gauge
will drop to empty when the car is turned off, but once it is started, the gauge climbs all the way above full. Questions: How many gallons does this tank hold? What is the average MPG's? Is there any way to trouble shoot this myself or is there a simple fix?
Any help would be great!
Regards, Rick
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