When I bought my tires from Costco, I noticed the caps for the valve stems were a flourescent green and asked about them - it means the tires are filled with Nitrogen. After reading a friend's blog claiming that nitrogen-filled tires get 0.5 mi/gal better gas mileage and have a longer tire lifespan, I wanted to find some "proof" to these claims. The belief, also pushed by Costco in their tire brochures, is that the larger nitrogen molecules don't react to temperature changes (better tread life and mileage) and lack moisture (won't corrode the tire from the inside). It doesn't affect my decision if these claims can't be proven because there is a miniscule increased cost for using nitrogen (I can only "fill" my tires at a Costco, most gas stations only have air, but the service is free).

The theory is that the oxygen in the air inside the tire, when heated as a result of driving, can oxidize and rot the inside of the tire over time. A nitrogen filled tire won't oxidize for obvious reasons, no oxygen. This is all anecdotal though and for most people, I think the tread of the tire wears out before the tire does from internal rot.

As for the nitrogen not reacting to temperature as much as compressed air, it's probably a bit of snake oil magic going on there. At such low pressures, the nitrogen behaves like an ideal gas (just as oxygen and water vapor) so that means pure nitrogen will act just like regular air. A 0.5 mi/gal improvement in gas mileage is 1.6% improvement (on my car, which gets a little more than 30 mpg) which seems way too much.

It sounds like the "nitrogen is better than air" theory is not entirely true but since it costs me very little (if at all really) I won't be complaining much about it. As long as you don't pay for nitrogen, fill it with nitrogen; but if it costs you money, ask for regular air.

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