Why or what is causing my gas meter to start off full and then go to empty?

-if i read my gage right i only have a 5 to 8 gallon tank in my 1997 mercury tracer. no leaks in the fuel system but the gage shows a quater of a tank and when i fill it up it takes 5 gallons to 6 but its rated for a 12.7 gallon tankhave it checked out. your gage must be broken