My fuel hand is on the full mark and the tank is not full how do I fix this problem? 1988 mustang

 

Wiring for the gauge could be grounded out, but its more likely that the sending unit is bad and/or the FP baffle broke loose and destroyed it.

 

My suggestion is that if it's the sending unit, you might as well replace the fuel pump all together. I have the same problem with my 88. If your going to drop the tank and take up your life doing that you might as well replace whatever you can so you don't have to do it in the future.