Why inflate auto tires with nitrogen?

Inhibits corrosion, maintains pressure as it has a different rate of diffusion (leakage) through rubber than normal air. After five years if the seals and valves were perfect (meaning never) nitrogen would loose about 8 PSI and air would loose about 11 PSI.

The primary reason many modern car tires are inflated with dry nitrogen is because ordinary air contains a small amount of water vapor, which in cold weather condenses out slightly reducing the tire pressure. While this slight reduction in pressure is not a problem for either the tires or vehicle safety, the electronic tire pressure monitoring systems that many modern cars now have are able to detect it and give a false low pressure warning, sometimes causing multiple visits to the dealership and owner frustration. As dry nitrogen has no water vapor, its use instead of air is a quick easy and inexpensive fix that results in an almost instant improvement in customer satisfaction.

I know I sure spent a lot of time that first winter after I got my first car with a tire pressure monitoring system. I was continually getting false warnings, spending time doing walk arounds trying to see a low tire, measuring the pressure with a gauge and not finding any tires low, topping off the tires with more air only to have the warning happen again a week later, etc. It was horrendously frustrating, I was even in the dealership a couple times to have them check the tire pressure monitor (which they were nice enough to not charge for as they found no problem). The next year they mailed out coupons for filling the tires with dry nitrogen. I have only had one problem with it since: about 2 years after the initial dry nitrogen fill I got another false low pressure warning on the first cold day of fall and the dealer topped off the tires for free. So I guess that maybe every couple years the normal slow loss of pressure requires a top off. My new car came with dry nitrogen "free", as part of a dealer option package.