Fuel Gauge Problem?

[Philossifer]

My fuel gauge is acting kinda’ strange... It doesn't read the amount of fuel evenly in relation to miles traveled versus gas used.

For instance, this past weekend I drove about 280 miles round trip. 140mile there, 140 miles back, no driving in-between. On the trip there, my fuel gauge dropped from full to just over ¾'s of a tank but on the way home, it used the remaining ¾’s of the tank.
The only thing I’ve done was adding a new fuel pump, but that was about two years ago and the problem seems to have surfaced only recently.

Any Idea’s?
Thanks in advance!
Phil
Forgot to add, it's a 1989 Mustang LX, 5.0L