Q: At the car wash yesterday, I overheard two car guys debating the pros and cons of using nitrogen to inflate their tires, instead of air. I don’t know (or care) much about cars and I like to keep things simple, but you know how it is when you’re trapped in a waiting room! Some of the things the nitrogen guy was saying made sense. On the other hand, air is free! Those two never came to an agreement but they got me wondering whether I should get nitrogen the next time I get new tires (which will probably be this year). Your advice? — Not A Car Gal
A: Great question, Not A Car Gal! I’ll bet I can list the points those guys were debating, and it is an ideal opener for my “Nitrogen 101” micro-seminar. Nitrogen-filled tires have been used on aircraft, heavy equipment, and high-performance vehicles (aka race cars) for decades. For the rest of us, is it worth it?
Nitrogen naysayers love to point that air is free, and of course they’re right. But tires and fuel are not free. Keeping tires optimally inflated can improve gas mileage and prolong the life of the life of the tires. How much difference does it make? That depends on several factors, some of which change over time. With enough information, each driver can decide what makes the most practical sense for their situation.
The air we breathe—and traditionally use to fill our tires—consists of 78% nitrogen, 21% oxygen, and 1% miscellaneous gases. It works fine but in several ways, using “pure” nitrogen works better. (By “pure” we mean 93-95% nitrogen—100% purity isn’t practical or necessary.)
Nitrogen is dry. It does not support moisture. Air always contains moisture, the exact amount depending on the ambient humidity and how effectively the air compressor dehumidifies the air before pumping it into the tire. The presence of moisture means oxidation can occur over time, affecting the rubber and even the wheel. And speaking of air compressors, some are cleaner than others, meaning the air inside your tires almost certainly contains traces of oil, dust, and other particulates. These substances can also contribute to oxidation and other processes that make tires turn brittle and wheels rust.
Moist air is more sensitive to temperature changes than dry air, changing pressure more rapidly and to a greater degree. Tires heat up while driving, and get cold when parked (even freeze in extreme conditions). This can affect handling on the road as well as tire life and durability. How much real difference does it make? Again, it depends on what kind of tires you have, how you drive and under what conditions, the climate where you live, etc.
Every car owner knows that tires gradually lose air over time, as molecules of air pass through microscopic pores in the rubber. Tests have shown that nitrogen-filled tires maintain inflation pressure somewhat longer than ones filled with air. (Nitrogen molecules are larger in diameter than oxygen molecules, so they take longer to leak out. In a future post, we’ll get geeky on this subject and explain why your tires are actually big bundles of spaghetti.) We are all advised, and rightly so, to check tire pressure once a month. How important is it to have nitrogen instead of air? It depends on driving conditions, parking conditions, whether you drive every day, or your car sits for prolonged periods, as well as the type of tires installed. And, of course, whether or not you check your pressure every month.
So, what’s the bottom line? That’s what your pals in the car wash waiting room were arguing about. Nitrogen has several potential benefits over compressed air but it’s extremely difficult to gauge how much difference each one makes to a specific driver operating a particular vehicle under whatever conditions pertain. Personally, I place a high value on what some call the law of incremental gains. Another way of putting it is, every little bit helps!