Just had the same thing happen with my 2015 Street Glide Special. I was on the interstate and actually looking at my gauge when it suddenly went from 3/4 tank to full. I took it to my mechanic and he checked the sending unit which appeared to be ok, so he ordered a new gauge. When the new gauge came in, he plugged it in and started going through the calibration procedure. When it got down to about half tank, he heard an electrical pop sound and the new gauge went to full. He called the company he got the new gauge from and talked to an engineer who said the sending units can have a short along the travel of the potentiometer and when the float gets to that point, it will short out the gauge. At that point the gauge is shot. My mechanic started ohming out the wiring and found a wire shorted out between the sending unit and the computer. Once that was fixed, the sending unit and the gauge had to be replaced. they had both shorted out. He also found that the shorted wire changed colors under the fuel tank. Harley had evidently run out of that color wire and just spliced in something else under the tank. I would guess that your gauge is shot. Ohm everything out real good before putting another gauge in though.