Did a search, haven't found the answer to this issue. From all the research I've done, it appears that a 1999 Tahoe 4-door should have a fuel tank of roughly 112L. When my fuel gauge is reading just under 1/4 tank, I fill up, and it only takes about 50L until it's full. So from what I can tell, the fuel gauge is treating a half tank like a full tank. What could be causing this? I'm guessing the float or fuel sender, or whatever it is in the tank that measures the fuel level just isn't working properly. On the plus side, my old Tahoe is getting better mileage than my new truck.