I have a similar problem with my 2013 RSX Sport. The Gauge will read full when the tank is full, but drops to 3/4 or 5/8 within a few miles of riding. I have been checking it with an old fashion measuring stick that I made up and the gauge typically will read much lower than the stick reading. When the gauge reads empty, the stick reading is 1/4 to 3/8 still.