Unplug the cable from the fuel gauge ... test with an ohm meter and post your findings.
I spent countless hours to figure this out as I had a similar problem. No information is posted online....
My 2006 sporster, similar to yours - use a reversed sending unit. Our Ohm reading is 0 - 90 ohms. ...
Normal cars, boats etc typically have 0 = empty, 90 = full. Ours are the opposite... 90 = empty, 0 = full.
If your ohm reading is at 0 (or under 5), yet your fuel is low, it means your baffle is stuck (there is a float, similar to the one in your toiler tank, where it connects is a little board and a needle that moves and gives you different resistance reading)
If your ohm reading is something else, then it means the problem is with your fuel gauge. - either 1 - not getting power, 2- stuck, 3 - worst case scenario (my problem) dead... very rear but can happen.