I too thought I was getting horible mileage in my '02 yukon with 115k. I generally fill up when the Low Fuel Light comes on. Just to give me an accurate reference point. Now I've noticed that when I fill up soon after that light comes on, I can usually only pump about 4-5 gallons shy of what the stated fuel tank size is. This leads me to believe that when the low fuel light comes on, you still have 4-5 gallons left. I've been averaging 19.5 on gas and 16-17 on e85.