Just got a new set of Michelins for one of our GMC 3/4 ton vans at Costco last week. When I got home I noticed all of the valve stem caps were green. Do you know why?

Is there a prize for the correct answer?

My guess is Nitrogen is in your tires signified by the green cap! "The science is simple: 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.
Nitrogen leaks out of tires about 30 percent to 40 percent slower than air, according to Ingersol-Rand Co., which makes nitrogen inflation systems. And it doesn't hurt anything to pump regular air into your nitrogen-filled tire.

If there was a prize, Richard would have won. What a detailed description of nitrogen. Thanks, you were right on target. I am a big fan of Cosco because of little things like that.

Thanks Rambo, I saw a news story on the subject a few weeks ago. They interviewed the manager of a Costco and they showed the nitrogen inflation system and how it works. I never really thought about it and didn't realize that air (or nitrogen) slowly leaks through the pores of rubber. I thought the story was interesting and researched it a bit.

Thanks Richard

First off your post was great and it got me thinking a bit. As a kid I raced BMX and road reasonably decent road bikes, three months ago I started riding road bikes again. Road bikes don’t have the traditional (Schrader) valves that our car tires have; they have what are called presta valves. The pain in the neck is that about every two days you need to top off your tires a bit. Normally I ride with about 120 psi. I wonder how much it would improve going with a nitrogen pump. And while we are on the subject I wonder if there is a weight difference?

David Gelinas
Marbleguy

Because Nitrogen molecules are bigger than air molecules they, do not expand as rapidly as atmospheric air. This is a great advantage in a racing tire. As heat builds up in a tire the air pressure increases. Thus changing the size and shape of the tire. With Nitrogen this problem is almost eliminated. That way the tire remains constant in shape and size. Giving the car a more consistent run lap after lap.