fuel gauge

The fuel gauge of my 91 PA is off. It always shows about 1/4 to 1/3 more than it should. So if you fill the gas up it'll stay on full for a long time, and when it points to 1/4 it is already empty.
How can this be fixed?

I bet it needs a new sending unit. The sending unit is a variable resistor. I have read that the sending unit/fuel pump can be accessed through an opening in the trunk for the Park Avenue, so that the tank does not need to be dropped. Hopefully, this is the case for the 91's as well. I had to drop the tank on my old Lesabre to get to it. I put in a salvaged one, that included the fuel pump as well. If you can pull it out, you could use a digital volt meter and measure the ohms as you move it back and forth. This would tell you for sure if you need a new one.

The 91 model has the tank about in the middle of the car. Would there be an opening too somewhere? When I took out the rear seat I don't recall seeing any access holes in the bottom.
I think you would have to drop the exhaust as well to drop the tank, so that seems too much work for such a minor issue.

I recall reading that some had access under the back seat. This may have been for the later model Lesabres though. The exhaust had to be dropped on the Lesabre, but it wasn't a big deal. Had to remove one bolt in the back and drop it all down from a rear hanger for clearance. The pipes did not have to be disconnected on the Lesabre (not sure on the '91 PA, but could be similar). The bolts for the tanks straps will probably be rusted though and may strip out. If so, you might have to cut the straps and then the bolts after the tank is out of the way. The straps could probably be welded back together with some reinforcement.

The 91-96 PA's do not have that access and the tank does have to be dropped..