Fill tires with air or nitrogen?

My friend Raymond puts nitrogen in his car tires. I use air. Naturally, we have had some science chats about which is better, especially for the best gasoline mileage.

There are a few basic science facts in this pressurized mix: Raymond uses elemental nitrogen gas, N2, molecules of two nitrogen atoms. I use "air," the common word for the gaseous mixture of our atmosphere.

Air contains about 78 percent nitrogen, 21 percent oxygen, and less than 1 percent argon. Other gases are about 4/100 of 1 percent. Air from the pump at my gas station also contains water vapor, molecules of H2O. The amount depends on the humidity that day.

So the questions are: What are the pros and cons of filling car tires with pure nitrogen, instead of 78 percent nitrogen and 22 percent other stuff?

There is one compelling reason to use nitrogen in tires. Under extreme conditions, such as aircraft and space shuttle tires during landing, pure nitrogen reduces fire hazard. Pressurized oxygen accelerates a fire in an overheated tire.

Since most passenger tires do not pose a fire hazard, then why bear the extra expense and trouble of filling tires with nitrogen?

Nitrogen molecules are larger than oxygen molecules, and thus do not leak out of the tires as easily. Oxygen and water vapor deteriorate the tire rubber, so nitrogen-filled tires and metal rims last longer.

Also, air is readily available at service stations, often at no cost.

For all drivers, tire pressure significantly affects gas mileage and tire wear. A tire fully inflated to the manufacturer's recommendation has less rolling friction than an under-inflated tire. It gets the best gas mileage.

Lower rolling friction also reduces heat, damage and wear and extends the life of the tire.

So, is leakage a problem? Consumer Reports conducted an experiment. They filled 62 tires, 31 with air and 31 with nitrogen, to a pressure of 30 pounds per square inch, and let them sit for a year.

They found that the nitrogen-filled tires lost 2.2 pounds and the air-filled tires lost 3.5 pounds in a year. The gas molecules passed through the tire over time. The difference is small, but noticeable.

A much more important factor than air vs. nitrogen is this: Under normal driving, tires lose pressure from many other conditions, including damage from rocks and glass. Checking tire pressure is far more important than which gas is used.

The two gases, air and nitrogen, perform similarly in tire operation. The oxygen and water vapor in air do react with the chemical components of the tire, and age it faster than nitrogen does. Water vapor in the air used to fill tires might condense in cool weather, reducing overnight tire pressure more than with nitrogen.

Bottom line: If you can fill your tires with nitrogen at low cost and high convenience, there should be a marginal improvement in tire life. If you check your tire pressures weekly, however, you will gain a much larger improvement in gasoline mileage and tire wear than the benefit of nitrogen over air.

Virginia's science Standards of Learning cover gas properties beginning in second grade (2.3) and continue in 5.4. In 6.6, students study the properties of air as a mixture of gaseous elements and compounds.