When it comes to tire-related trends, there are few as widespread and long-lasting as the idea of filling your tires with nitrogen gas rather than standard compressed air. People will tell you that nitrogen-filled tires will lose pressure slower, react less to temperature changes, cause less rubber degradation and make you more attractive to the opposite sex. Okay, maybe not that last part, but I wouldn't necessarily be surprised.
Do keep in mind, however, that the people who are saying this are generally those who have something to gain by selling you nitrogen at a premium. So the question is: Is it really worth the money to fill your tires with nitrogen. In general, I would say that while the benefits of nitrogen are actually real, the answer to whether those benefits are worth the costs is almost always no.
Here's the issue: The compressed air that fills most tires has a few major disadvantages.
- Oxygen molecules can escape from rubber tires by slipping between the molecules of rubber. It is estimated that up to one psi of pressure is generally lost for every month the tire is inflated.
- Oxygen is a reactive gas that easily combines with many other materials, such as metal and rubber, and this process of oxidation works to degrade those materials. In metal, we call this process “corrosion” or “rust.” In the layers of rubber that make up tires, we call it “tire aging” as the rubber breaks down and becomes stiff and brittle.
- Oxygen will support moisture formation by combining with hydrogen, and compressed air will almost always contain some water anyway, increasing the risk of corrosion.
- Air-filled tires will react to temperature changes in a big way, usually with pressure changes on the order of one pound per square inch (psi) for every 10 degree change in ambient temperature, and even more pressure changes in response to operating temperatures as the tires heat up while being driven.
Nitrogen gas, on the other hand, supposedly solves or diminishes all of those problems.
- Nitrogen molecules have a much more difficult time escaping from tires, reducing the loss of pressure over time by about 1/3 or more.
- Nitrogen is much less susceptible to pressure changes as a result of operating or ambient temperatures.
- Nitrogen is an inert gas, meaning that it does not react or combine with most other materials and so does not cause corrosion.
- Nitrogen is a “dry” gas that does not support moisture formation.
For these reasons, nitrogen has long been the gas of choice for filling critical tire applications such as racing tires, aircraft tires and heavy equipment tires. The advantages of nitrogen in these applications are obvious and unquestioned. It's when you get into using nitrogen for your daily driver that the questions really begin.
The most important question is: How pronounced are these differences between air and nitrogen in tires? The answer: Not much at all. After all, even “standard” air is composed of 78% nitrogen to begin with!
In 2006, Consumer Reports conducted a study to determine how much pressure tires filled with nitrogen lost versus those filled with air. The study showed that the difference in pressure loss between nitrogen and air amounted to 1.3 psi over the course of a year. From an initial pressure of 30 psi, air-filled tires lost 3.5 psi, while nitrogen-filled tires lost an average of 2.2 psi. Nitrogen won, but not by much. A 2008 ExxonMobil study looked at changes in temperature over various inflation pressures, and found that the differences between nitrogen and air-filled tires were incredibly small.
As to corrosion due to oxygen reactivity or moisture inside the tires, this is simply not in any real way a problem with modern wheels, which are clearcoated to prevent just such corrosion. Even if the clearcoat gets broken or scraped, this kind of corrosion happens so slowly that your car would probably wear out before metal corrosion on the wheels ever became a noticeable issue. And while nitrogen-filled tires will prevent oxidation of the tire rubber from inside, it doesn't do anything about oxidation from outside.
So in the end, when talking about consumer cars on normal roads, there really is almost no difference between nitrogen and air-filled tires. Even with nitrogen, you still have to check your tire pressure regularly, since even TPMS monitors will only show when your pressure is critically low, not just a bit low. You still need to pay attention to major ambient temperature shifts. You still don't really need to care about the possibility of corrosion, because it's negligible no matter what. The hype surrounding nitrogen is pretty much just that – hype.
Compare the almost invisible advantages to the very real disadvantages. It's difficult to find shops with nitrogen rigs – most major tire shops don't have it. Most shops that have nitrogen rigs charge $5-$10 per tire, which is pretty stiff compared to 75 cents at a gas station for air. If any of your tires lose pressure, you must either find someplace with a nitrogen rig and pay again, or top off temporarily with air, which will require a complete bleed-and-fill later when you can find someone to do it.
Tiny differences in heat-cycling pressure can mean a lot when you're filling up your IndyCar tires, but if you're paying a $5-10 premium per tire to fill the Ecopias on your Honda Civic with nitrogen, then it would be my strong opinion that you're getting ripped off.
For a much deeper look at the differences between nitrogen and air in tires from an engineering perspective, with actual numbers and equations and stuff, I cannot recommend the Barry's Tire Tech web page highly enough.