If we looked at it from a pressure stability perspective, air has just a slightly higher specific heating value. This means that it takes just a little more energy (btu) to increase the temperature of the same amount of gas by one degree. If it takes more energy (even though its splitting hairs) to heat up air than Nitrogen, then that means the same heat generated at the tread would increase the temperature of the Nitrogen gas just slightly more. This would then translate to the fact that Nitrogen fill tires should increase with pressure more than air fill tires.
So, who in their right mind would put Nitrogen into their tires instead of free cheap air? What we've ignored so far is water, or water vapor, to be exact. All of the calculations so far were based on the assumption that air is dry, which is far from the case. The amount of water in the air greatly varies from 4 percent to its lowest of 0.5 percent. When large amounts of water are present in the air, other elements are present in lower amounts, which lowers the overall density and weight of air since water has a lower molecular weight than Nitrogen or Oxygen, the major components of air.
If you've ever worked around a compressor, you'll know that under pressure, water collects at the bottom of the tank. This comes from humidity which, under the pressures inside a compressor, becomes liquid water. When that's pumped into a tire, it immediately expands back into vapor due to the much lower pressure inside the tire.
Unfortunately, the conditions inside a tire make it very easy for water to change between gaseous and liquid states since it has such a low vapor pressure and boiling point. Under cold conditions, the humidity inside the tire takes up very little space. Heat it up to full operating temps, and the water vapor expands and increases pressure far more than dry air.
This is where the real world advantages of Nitrogen come from. Nitrogen in a bottle is free of moisture, unlike the air you get from a compressor. Nitrogen molecules are also much larger than Helium and water molecules, which are present in air and can permeate through rubber causing pressure loss over time. The low permeability of Nitrogen and lesser effect of heat on tire pressure make Nitrogen filled tires require less service and allow racers to get out on track and be at full operating pressures faster. High speed supercars like the GTR also don't have to worry about pressure changes from the extreme heats of high speed runs. Some claim the slightly lighter weight of Nitrogen also reduces rolling resistance and rotational inertia for better fuel economy. How much is your guess?
But there are some practical disadvantages. Filling pure Nitrogen into a tire isn't easy since you have to get all the air and moisture out. Most race teams with single valve stem wheels have to fill a wheel and then purge it three times to get it close to pure Nitrogen. That can get costly. Some wheel manufacturers like Enkie make a dual valve stem wheel like the NT03+M where one valve is use to fill with Nitrogen, while the other valve is for simultaneously purging the air. It might have been better if they placed the stems 180 degrees from each other for better purging instead of adjacently.
Is Nitrogen better? Maybe, but I'll stick with air, which is almost 80 percent Nitrogen. And being in the low humidity desert climate of Southern California means that the air in my tires isn't mostly water. The minuscule weight and rotational inertia gains are a questionable matter since you'd end up with a lot better fuel economy by just bumping up your commuting tire pressures by 5 psi.
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