I found this on Nascar web site. Most of the teams remove the air from the tires and replace it with nitrogen. Compressed nitrogen contains less moisture than compressed air. When the tire heats up, moisture in the tire vaporizes and expands, causing the pressure inside the tire to increase. Even small changes in tire pressure can noticeably affect the handling of the car. By using nitrogen instead of air, the teams have more control over how much the pressure will increase when the tires heat up. I was not able to talk to a mechanic at work they were to busy how ever the nitogen bottles use to fill aircraft tires are labeled compressed nitrogen un 1066. Hope this helps. :patriot: