Tires with green caps are filled with nitrogen, instead of compressed air. Nitrogen is a naturally occurring, colorless, odorless, tasteless gas that makes up 78% of the earth's atmosphere. The vast majority of tires are filled with compressed air. Nitrogen-filled tires are a recent development and are proving popular with consumers in climates that have large shifts in temperature during the course of the year.
Nitrogen-filled tires have green caps so that everyone will be able to recognize that they are not filled with air. There are two major benefits to these tires: they reduce the rate of gas loss and they eliminate moisture from within the tire. Nitrogen-filled tires are slightly more expensive than standard tires due to the cost of nitrogen and the benefits the tire provides.
Tires filled with nitrogen retain the optimal pressure levels for longer than air filled tires. This creates a more even wear pattern in the tire and provides better gas mileage. Nitrogen molecules are fatter in shape than oxygen molecules, and as a result, they have a lower leakage rate through the tire walls. These walls are porous in order to allow the rubber to expand and shrink as needed.
Humidity is the level of moisture in the air. Moisture is held in oxygen molecules, so a tire with no oxygen, such as a nitrogen-filled tire, is not affected by humidity. This difference results in a more consistent tire pressure.
The fastest way to improve gas mileage and extend tire life is to ensure that tire pressure is checked regularly and is at the appropriate levels. The National Highway and Traffic Safety Associations (NHTSA) reports than less than 60% of American motorists regularly check their air pressure. Nitrogen-filled tires retain their pressure level for longer, providing a lower maintenance product than standard tires.
Over time, the nitrogen levels will decrease, and more nitrogen will need to be added. Drivers can contact their local automotive or tire dealership to fill the tires with the correct gas. Filling the tire is usually a fee based service, with a flat rate per tire. The air pump nozzle is a different shape than the nitrogen pump. This is to ensure that tires with green caps are not given oxygen in error.