Fuel gauge says I need gas when I dont

I have a 2009 with 74k miles on it. I started having an issue with the fuel indicator gauge. My MPG still shows upwards of 40 when I check it, however, the gauge drops and begins the 1 pip blinking at about 250 miles. I when to fill up thinking maybe it was bad gas, but it only took about 4.5 gallons. The gauge returned to full. 250 miles later it repeated and this time I paid attention and again only could put about 4.5-5 gallons of gas. I looked up how to reset the calibration and tried it a few times to make sure I didnt mess it up somehow. It is still doing it. The MPG still says upwards of 40 MPG. So this time I pushed my luck and drove it 100 miles when it started blinking. It still only took 8 gallons when I filled it up. So it is clearly the gauge. Any suggestions besides the $129.00 diagnostic fee?