I've been having this problem for some time.... When i fill my tank up and i start driving, the fuel gauge will read above the F mark, and then all of a sudden trip out and drop to a hair above half then go up again. I just went to the snow, and i noticed that as soon as i got to about 6000 ft it started doing it at half tank. The only reasonable explanation i can give myself is that the fuel cap is screwed up and is not letting air in to compensate for the fuel that is being removed. The vacume in the tank would cause the atmospheric pressure to drop thus making the fuel float heavier causing it to drop in the tank...or some scientific reason like that...Just wondering if anyone has had the same problem with thier's....