Nitrogen has been used in aviation tires for decades for a good reason. They use Nitrogen at COSTCO in their tire department for the same reason, otherwise, why bother using NITROGEN at all.
Why bother using nitrogen at all? To begin with, if you live on Planet Earth, nitrogen is a part of life. It's unlikely you'll escape it....
OK, one more time on this Nitrogen thing….
Q: Why did Aviation adopt Nitrogen fill?
A: An aircraft descending from 40k ft to sea level can experience a 150’ F temperature excursion in a very short duration, way too fast for the tires to catch up. That could put them at up to 15 lbs low when the wheels touch down. Anything the industry can do to help stabilize the pressures greatly reduces tire wear or loss. It’s unlikely your car experiences these rapid changes.
Aviation Best Practices Fun Facts: They reduce the impact of initial water, do repetitive pump-&-purge to remove oxygen & water vapor, and use PURE nitrogen from a LN2 source.
Q: Is nitrogen more stable over temperature than oxygen? First order answer is NO. Remember the Ideal Gas Law from freshman chemistry? As long as they remain a gas over the temperatures considered, they expand and contract at essentially the same rate. The outlier is water… It changes phase and thus pressure contribution. You know that latex stuff they slop on your bead to make mounting and sealing easier? It’s mostly water that gets left in your tire. It contributes a partial pressure when the tire is hot, condenses to liquid when cool, and could freeze on a winter night. Fortunately, water leaves the system quickly via diffusion. But until it diffuses out, your tire pressure could be unstable. Have you ever seen liquid water in a dismounted tire? Highly unlikely…
Now what do you get at Costco?
How much gas goes into your tire? Technically about 3.4 atmospheres. The first atmosphere is the room air that fills the empty space when the tire is placed on the rim. It contains about 78% nitrogen. You then fill to 36 psi (14.7 psi / atm), or about 2.4 atm. So almost a third of your fill is room air.
Does Costco give you 100% nitrogen? Some tire stores, when nitrogen was first introduced, had real liquid nitrogen tanks out back, filled by an Air Products (or other supplier) truck. Today, most use Nitrogen Concentrators. These are similar to the Oxygen Concentrators used at home by heart failure patients, only with the osmosis filter & pump system running in reverse. They deliver about 95% nitrogen. Does Costco do a multi-step pump and purge prior to filling to remove the air?
My math says you have a net initial fill of around 92% nitrogen in your tire from Costco. Yes, that’s better than 79%, but it’s not magical. Your tire is a natural sieve. The first stuff to leak out is the smaller molecules: water, oxygen & carbon dioxide. What do you have in your tire a month later, three months later? Both tires are probably approaching 95 – 98% nitrogen purity on their own. Even if you topped off every few weeks with house air. It’s unlikely you will experience ANY functional difference three months or so into your new tires.
The one thing I would advise – If you have a home compressor, make sure you drain the condensed water out occasionally. Water is far more your enemy than oxygen for a number of reasons.