I have the same issue with mine, and have been told that GM is working on a fix for this. My truck also has a 26 gallon tank and my low fuel light comes on at 16 gallons burned. When this first happened, I finially got tired of it and asked my dealer to look at it. They told me that it was not throwing a code, and would be $250.00 labor bill if nothing was worng with it. I asked if they found 10 gallons of diesel would they agree there was a problem and they said yes, so I told them to do it. There was just over 10 gallons of diesel still in the tank. They replaced my fuel sensor in the tank under warranty and this fixed the problem for one month and now it is back doing the same thing again. The fuel level burned is accurate, but does not let me utilize all the fuel in the tank. Hoping there is a fix for this soon.
2008 GMC 2500HD LMM SLT CC SB 4x4 Z71
Buckstop front bumper, WARN M12000 winch, PIAA 520 ATP driving / PIAA 510 fog, Westin Platinum Series nerf bars, Pace Edwards Bedlocker tonneau cover, Cognito UCA leveling kit and HD Idler Pivot Assembly, Cognito Pitman and Idler steering braces, Bilstein 5100's, LT285/70R17 BFG AT KO tires, 17 x 9 XD Series Crank Wheels, 15% Tint Driver/Passenger windows, GM Tow Mirrors, DDM Tuning HID lights 6000K 35W