RichardL,
OK, last time I'm posting on this.
You have made exactly my point. Nitrogen will leak out of valves and pinhole leaks or any other leak caused for any reason just like air. I feel these are a much bigger contributer to pressure loss than permeation will ever be. So why pay the extra money for Nitrogen, which is much more than the 25-50 cents people have mentioned for air. (And frankly, I still find it for free).
Temperature will have a different effect for air vs. nitrogen due to the difference in water levels for each gas. The effect is pretty much negligible, however, so hardly worth discussing. Racing does use it for this reason, but it's a bigger deal when you're doing 180mph and tenths of a second count.
My understanding as to why airplanes other items use nitrogen has more to do with convenience than anything. I believe (and I admittedly could be wrong about this) that higher pressures are required in many aircraft tires, etc. and it is easier and cheaper to do this with nitrogen than straight air. Again, this is due to water content.
By the way, below is an excerpt from an article in the St. Petersburg Times citing Michelin officials.
At least one tire manufacturer is even more skeptical about the advantages of nitrogen in the family car.
Michelin officials recommend nitrogen only for tires used "in a high risk environment and/or when the user wants to reduce the consequences of a potential abnormal overheating of the tire-wheel assembly (for example in some aircraft applications)," according to a company statement.
But for all other tires in normal use, nitrogen "is not required and does not necessarily bring the expected benefit.
"It is true that the physical properties of nitrogen reduce the pressure loss due to the natural permeability of the materials of the tire and thus the broad use of nitrogen will in general assist motorists with pressure maintenance.
"Nevertheless, the existence of several other possible sources of leaks (tire/rim interface, valve, valve/rim interface and the wheel) prevents the guarantee of better pressure maintenance for individuals using nitrogen inflation."
That's it. I'm done.
Rocks