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Sunday, November 15, 2009


By JC Leahy

Maybe I was ripped off. You decide. I went to Castle Ford for a "free" oil change. A year's free oil changes had come with my new car. While I was there they gave me a sales pitch for putting nitrogen in my Mustang's tires. It sounded like a novel idea and seemd to make some sense, so I said okay. That cost me around $70. It was a year and a half ago.

Since then, I have done a little research. Replacing air with pure nitrogen in tires was not exactly a novel idea -- just new to me. Nitrogen in tires is common in race car driving and commercial fleets. For race cars, small changes in tire pressure are really important. Race car tires get warm from friction as the car races around the track. Normal air contains water vapor -- humidity -- which expands and contracts a lot more than pure nitrogen. If small changes in tire pressure are important to you -- as they are is in NASCAR racing -- nitrogen makes a lot of sense. On the other hand, if you're just commuting to work, you might not care.

For commercial fleets, nitrogen has other advantages. First, although normal air is 78% nitrogen, it also contains 21% oxygen, a little CO2, a little water vapor (humidity), and minute amounts of noble gasses such as neon and argon. Nitrogen molecules are relatively large. Because of the larger molecular size, the theory is that nitrogen won't seep through the tires as fast as oxygen. That means that tire pressure will be more stable over time, leading to improved fuel efficiency, better tread life, and predictable handling (safety).

Also, because pure nitrogen doesn't contain water vapor or oxygen, there is no oxidation of the tires or the aluminum or steel rims. After all, you can't have oxidation without oxygen. Low oxidation is supposed to reduce tire aging and wheel corrosion.

The National Transportation Safety Administration has confirmed that nitrogen does, indeed, reduce tire aging in commercial fleets. Personally, the tread of my own tires has always worn down long before aging of the rubber ever became an issue. Whether or not reduced tire aging would benefit you is something for you to decide. There is some theoretical benefit.

Consumer Reports conducted a study to determine if nitrogen-filled tires held pressure better than air-filled. They filled 31 assorted tires with nitrogen and 31 matching tires with air. Then they put them outdoors for a year and checked the pressure afterwards. Air-filled tires lost an average of 3.5 psi and the nitrogen tires, 2.2 psi. So, yes, nitrogen tires will hold pressure better. The Consumer Reports study was flawed because their 62 tires were not mounted on cars and driven -- they were just sitting there. It is possible that under actual use, the relative benefit of nitrogen would be greater.

Driving my own Mustang, I noticed no difference in handling between tires filled with air and tires filled with nitrogen. I did imagine I saw a little increase in fuel efficiency, but not much. My preception may have been wishful thinking.

I have concluded that the important goal is to maintain proper tire pressure. Proper tire pressure will give you safer handling, better tread life, and better fuel efficiency. Nitrogen in your tires will do no harm and may help your tires maintain pressure. Nitrogen is not, however, a substitute for regular checking of your tire pressures.

Bottom line: Get yourself a good tire pressure gauge, keep it in your glove compartment, and use it. Buy your tires at somewhere like Costco that will fill your tires with nitrogen for FREE and refill them with nitrogen FREE for the life of the tire. And, by the way, Costco will also give you a pretty great Costco tire warranty on your tires and rotate them free anytime you say, and not charge you anything for mounting and balancing. You can have nitrogen re-filling done while you shop. I would NOT again pay $70 for nitrogen in my tires -- I believe Castle Ford got me on that one. :)

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