Green-valve-stem caps aren't a fashion choice — they signify that the tire contains nitrogen instead of regular air. There are benefits to this, but it's not always easy to find a place to get ...

Traditionally, car tires have been filled with compressed air. Air is 78 percent nitrogen and just under 21 percent oxygen, and the rest is water vapor, CO2, and small concentrations of noble gases ...

Have you ever noticed cars with mysterious green valve stem caps on their tires? If you have, you may wonder why some people choose to use them. Don't worry — a lot of people have the same ...