The fuel sender is worn out.
It's not a difficult DIY job. You can possibly fix the old sender or order a new one and install it.
The sender has two thin metal contacts that wear out over time as they move back and forth across a circuit board. I bent them back down to contact the board and, viola, I had a properly working gas gauge again.
15 MPG is pretty low? Are you doing a lot of city driving? I'm getting ~17.5 driving in NJ/NYC.