One fact that many people may not be aware of, is that airplane tires and race car tires are not filled with air. They are filled with nitrogen. This is fast becoming a common practice in passenger cars and commercial vehicles for a couple of reasons. One of the aspects of maintaining optimum gas mileage is the maintenance of the air pressure of car tires. Tires leak air, this is normal. It is best to check the air pressure once a month, as this will not only ensure that your car is getting the best gas mileage as possible. This is also a safety consideration, as over or under inflated car or truck tires will cause the vehicle to handle differently, less efficiently. When nitrogen is used to fill car tires, this becomes less of an issue. Nitrogen does not leak from the tires as quickly as air leaks. Tires stay at their full capacity for longer stretches of time. This is a consideration of finances, as well as a consideration of many people and companies looking to move into a more "greener" way of life. Using less gas means wonderful things for the environment. Take into consideration studies of gas mileage in connection with air pressure. The Bridgestone Tire company conducted a study and found that in the countries of Europe, more than ninety three percent of the cars on the roads had under inflated tires. That is amazing to think that only one in ten cars was not wasting gas. This should be such an easy problem to fix. Most analysts and researchers have concluded that this situation is similar in North America as well. And when you consider just how many cars are on the road each day in the United States alone, it becomes amazing. So, the proponents of nitrogen filled tires have a case, a very strong case. There are centers located in many cities where you can take your car to have the air in the tires replaced with nitrogen. Many of them use a system called PurigeN98. The cost to maintain the nitrogen in the tires is just about forty dollars a year, which may be more than spending a few quarters at the air pump, but the benefits to the environment far outweigh that cost.