mine is very similar to yours. by the time my low fuel light comes on, i cant put more than 20 gallons, so i have a mysterious 6 gallon black hole. i never really pushed it too far beyond the light coming on since i dont carry spare gas. i think its pretty common for the guages to be way off. i had another chevy before my tahoe and that guage was way off too. i could to 200 miles on the first half of the tank, but only 50 miles on the second half of the tank, atleast according to the guage.