Green caps on tire valve stems usually mean the tires are filled with nitrogen instead of ordinary air. Car dealers, tire dealers and repair shops have touted nitrogen for several years as a better alternative to air based on claims that nitrogen doesn't leak as much, so tires stay fully inflated longer. The catch is that topping off the nitrogen typically costs from about $5 to $10 per tire, and the initial charge for filling the tires can be much higher, so it's also a revenue source for service outlets.
It is true that pure nitrogen doesn't leak out of tires as quickly as air simply because a nitrogen molecule is larger than an oxygen molecule, so tires should remain at or near their recommended inflation levels longer. Fully inflated tires last longer and improve fuel economy and road-holding ability. It's also true, however, that nitrogen will still leak out of tires over time, just not at the same pace as air.
Nitrogen proponents also argue that air contains moisture, which can build up inside tires and possibly corrode the wheels and tire pressure monitors, as well as accelerate tire rot from the inside. Some air compressors use driers that remove moisture before it gets into the hose you use to fill your tires, but many don't. Nitrogen, in comparison, is "dry."
On the other hand, a nitrogen-filled tire isn't 100 percent pure; about 93 to 95 percent of what's inside is nitrogen, but the rest is air. Ordinary air is 78 percent nitrogen, so the difference isn't huge, and compressed air is cheaper to use and still free in some places.
In addition, whether a tire is filled with ordinary air or nitrogen, it will leak if the tire valve is faulty, if the tire isn't properly mounted and sealed on the wheel, or if the tire has a nail in the tread or other damage.
The Rubber Manufacturers Association, a trade group for tire manufacturers, says nitrogen "may contribute to minor reductions in inflation pressure loss," but also notes that "use of nitrogen alone is not a replacement for regular inflation pressure maintenance.