Reply #27: Fill your tires with Nitrogen if you can.

[IChing]

Nitrogen molecules are bigger than air molecules, so they leak out more slowly and tires stay inflated longer. This inert gas, which makes up 78 percent of the atmosphere, has been used for years to inflate high-performance tires on race cars, military vehicles, giant earth-moving equipment, even the space shuttle." It is possible to buy a pump that filters nitrogen from the air, and some businesses like Costco and America's Tires have started filling the tires of their customers with nitrogen at no extra charge over a year ago.

http://www.treehugger.com/files/2005/10/inflate_your_ti.php