Gas gauge not working
 

I have a 1991 F250 5.8 with dual fuel tanks. I just filled up both tanks and when I switched over to use the other tank the gas gauge now is showing empty. I opened up the gas cap and it showed a quarter of a tank and now it’s showing empty again. Any ideas?

A little FYI,
On those years, if the fuel gauge wire was shorted to ground, the gauge reads full.
If there is an open between the sending unit and gauge, the gauge will read empty.

There could be a few reasons for the incorrect readings like if the float is full of gasoline it will sink to the bottom showing an empty reading. Same result if the float lever is stuck .The gauge could also be a problem. Now you know what produces what reading , you might verify wiring and connections. Don;t forget to make sure the tank switch is good. (If it runs on both tanks , sw is probably OK).
The last thing you want to do is drop a tank to verify sending unit is OK, BUT you may have to do that.
Wait until the tank is almost empty,they can get pretty heavy when full of fuel.