Originally posted by: walrus
Air is 78% nitrogen already. Race cars use pure nitrogen because it doesn't have any water vapor in it that can change the tire pressure slightly at high temps. But it won't affect your mileage significantly
Ding ding ding. Winnar!
It's amazing how much misinformation or poor logic surrounds the topic of Nitrogen in tires.
Nitrogen migrates through tire rubber slower than air does, but the difference is minimal.
The biggest benefit is it tends to be almost completely dry. Because of this, temperature does not change the pressure very much. Yes, it follows the ideal gas low (obviously), but normal air has a lot of moisture in it. That's what really affects the pressure, and is often overlooked by those only remembering part of the ideal gas law.
When you're using racing tires, or even running your DOT tires really hard, it's easy to shred them past a certain PSI. For instance, both the Pirelli Corsa and Michelin Pilot Sport Cup (both great DOT-R choice) fall apart pretty quickly above 36 PSI or so. With a cold pressure of just under 30 PSI, air with moisture rises over 36-40 PSI quickly on many tracks, while dry air and Nitrogen do not.
Now, if your local station has a very good system in their air pump to remove the moisture (many high end systems do) AND they are nice enough dump out the moisture reservoir every day or so, that'll give you the performance of Nitrogen. If not, Nitrogen is a good way to keep a consistant temperature for sporting use.
Note: Many of places do Nitrogen fill ups for free.
You do a few deflations/inflations, and you should be very close to moisture free when filling with Nitrogen. Some track wheels have two air valve stems, one to release air and the other to fill, at the same time.
Who is the sillier person? The person who gets their Nitrogen for free or near free at many local shops, or the person who shreds $1700 worth of tires in a single track day to save the few bucks?