Years ago I used to be a tuggie and flew a Piper Cub and a Citabria, the Cub had a left tank and right tank selector the Citabria an on/off selector, they were both gravity fed systems, the interesting thing is that the Piper Cub ran out of fuel 7 times and the Citabria not even once. The point is regardless of fuel burn the average pilot is an optimist and will happily run on low fuel if he/she thinks that there is another tank to switch to if the engine stops. In the Citabria if you ran out of fuel it was forced landing time. Does this mean the Cub has a lesser fuel burn? the pilots thought so!