I haven't noticed any fluctuations in mileage with the start or end of a tank - mine usually stays quite consistent given identical driving conditions - but I have noticed that the last 1/3 or so of the gas meter will disappear much more quickly than the first 2/3. Usually when the fuel gauge is reading 1/2 or so, I've already got 450 miles or so on a tank, and none of my tanks ever reach 900. That said, I'll reiterate that the FCD shows no change in MPG during that last 1/3.