My bet would be that the float in the gas tank is stuck, broken, i.e., not working correctly. The only way to fix this is to lower the tank and either replace or repair the float, which is the device that sends the signal for the fuel level to the guage on your dash.
I know this firsthand because mine has been like this for a little over a year. No big deal though, I just keep track of fuel by using the trip odometer. The gas tank holds 19.3 gallons, and figuring a ball park of about 15 miles to the gallon, I know I can go about 300 miles on a tank. Take the risk factor out of it, and I fill up at around 250 miles most of the time, which usually leaves about 3 gallons left in the tank - don't want to cut it too close.
Of course you can figure out the mileage you're getting by dividing the miles you've driven since last filling up by the number of gallons it took to fill up the tank. Miles per Gallon. This method always works well for me, I'm never sweating that I'm not sure how much fuel I have left. I've driven tens of thousands of miles this way, in and to very remote places. So if you don't want to spend the money yet to fix the problem, this method can be used in confidence.
