I can only fill my tank to 3/4 full according to the gas gauge(about 12 gal).
What could be the issue and how do can I fix it??

 

So, you can only put 12 gallons in from empty and the pump clicks off? Or when the tank is full the gauge only reads 3/4?

 

The pump clicks off at around 12 gal and my fuel gauge is only reading 3/4 full.
I'm also only getting around 110 miles to a tank at this point.
I also just recently changed my fuel pump.

 

sounds like your tank vent line is plugged. I would try to fish some wire down from the filler neck. I wouldnt do it in a way it could make sparks.

 

I've never worked on a '96 but if you dropped the tank to change the pump (which apparently you don't need to do) I can see the possibility of maybe kinking a vent line or something like that when putting the tank back in.

My grandfather did that on his Ranger and the pump would click off after a couple gallons unless you filled it very slowly.

 

The other possibility is that you have a bad ground. You have to have a good ground or the gauge will read low.