The far left device is the fuel gauge with a remote readout sending unit connection. The gauge should tell you how much is in the tank at the direct reading dial. If the inside gauge reading is different, then the sending unit may need replacing, but check for a second wire in the harness that should be connected to a ground point first.
If the gauge does need replacing, they're readily available on Amazon, etc, and it isn't necessary to empty the tank. The gauge is either held in place by two small screws or simply snaps in place. It's magnetically coupled to a float assembly in the tank.
