Originally Posted by BlazingPedals

If nitrogen really did leak out more slowly than other gasses, the 70% nitrogen in atmospheric air would naturally distill as other gasses leaked out of the tube, eventually leaving you with all nitrogen. But it doesn't work that way, does it? Nitrogen inflation might make a small difference in aviation, where getting rid of all moisture matters, but not in bike tires.

I've found that tire thickness also has a bearing on air loss. While it's the tube that mostly holds the air, apparently the tire walls can also help to impede leakage.