Nitrogen. It’s all around us – the atmosphere is in fact 78% N2 – but is it in your tires, too? Probably not.
In fact, most people may not have even realized it was possible to put anything other than plain, regular air (which, I might remind you is mostly nitrogen) in their vehicle’s tires in the first place.
Unless, of course, they’ve been indoctrinated into the Cult of Nitrogen, which hands out green valve stem caps and preaches that this inert gas is the holy grail of driving efficiency. Hypermilers and eco-oriented motorists alike have long adopted nitrogen as their tire gas of choice, eschewing ‘common’ air in favor of this much-lauded interloper. The received wisdom that nitrogen is better, full stop, than anything else you might put in your tires is frequently one of the bedrock beliefs for those seeking to reduce emissions and fuel consumption on a daily basis.
How many of nitrogen’s promises about efficiency and performance are gospel, and how many belong in the realm of automotive Apocrypha? The answers might surprise you.
Atmospheric Air, AKA ‘Almost All Nitrogen’
The crux of what has attracted so much attention to nitrogen from those who want to drive green, and the companies that want to exploit them, is inflation. Specifically, there’s a strong belief that a tire that has been filled with nitrogen won’t deflate at the same rate as one that’s simply filled with the mix of 78% nitrogen, 21% oxygen, and 1% ‘everything else’ that you get from a standard service station compressor.
Why does this matter? Tire deflation is a major contributor to wasted fuel. In fact, one tire maker study found that Europeans were wasting 2.14 billion gallons of gasoline a year as a result of riding on under-inflated tires (which is a term that described more than 90 percent of the tires rolling around that continent).
Tires that have been filled with atmospheric air tend to leak at a rate of up to 1 pound per square inch (psi) per month, which is why it’s recommended that you check your tires every few weeks to make sure they’re still properly inflated. Nitrogen proponents, however, argue that the slightly larger size of nitrogen molecules, as compared to oxygen, means that they are less likely to seep out through the rubber of a properly-sealed tire, dramatically reducing the rate of deflation.
While it’s true that nitrogen is easier to keep captured inside rubber, let’s circle back a minute to the composition of the air that we breathe (and that typically ends up inside our tires). It’s 78% nitrogen already, which means that moving to pure nitrogen increases the amount of that gas inside the tire by a mere 22%.
Don’t think that means the tire will stay inflated 22% longer than one filled with ‘regular’ air, either. All-nitrogen tires will resist deflation somewhat better than their atmospheric-filled equivalents, yes, but not at a rate that is likely to be noticed by the average driver. That means you’ll still need to check your tire pressures on a regular basis, only this time when you need to refill, you’ll have to head to a nitrogen-supplying garage and pay for the privilege. This can be as much as $6 to $10 per tire, depending on how much nitrogen is needed, as opposed to the usually free, sometimes drop-in-a-quarter cost of atmospheric air.
But What About Temperature?
Deflation is also an issue for tires filled with atmospheric air during the changing of the seasons when the hot weather gives way to cold, and vice-versa. When temperatures drop, so do tire pressures, which often leads to driving on underinflated rubber during the fall and winter months.
Nitrogen fans often point out that their favorite gas is able to resist the same type of deflation – except it isn’t, really. In fact, nitrogen will lose one pound per square inch of pressure for every 10 degrees Fahrenheit the thermometer falls (or roughly 2% of pressure). You still have to check your tire pressures during seasonal changes or cold snaps whether they’re filled with nitrogen or plain old air.
There’s a big asterisk that needs to be added to the above section, and that has to do with moisture. Although pure nitrogen loses pressure at very nearly the same rate as the atmospheric mix of nitrogen and oxygen does when bottled up inside a tire, it has the advantage of being almost completely dry in a way that ‘regular’ air isn’t.
Why is this lack of moisture important? The presence of water vapor inside a tire can lead to larger swings in pressure related to temperature. This is true when a tire gets hot after driving for an extended period of time (increasing pressures), or when it gets cold in the winter (decreasing pressures), and it’s because water vapor isn’t nearly as stable as either oxygen or nitrogen when it comes to resisting changes in temperature.
It’s tough to control how much moisture is present in an air compressor, and some will add more than others to your tires when they are being filled. Nitrogen, on the other hand, is close to 95% percent pure, which guarantees almost no moisture will be introduced to a tire while filling it up. This has a few other benefits too, including reducing the chance of rubber or alloy corrosion on a wheel or tire over time.
Not Worth The Cost
Even taking moisture into consideration, however, it’s hard to assign nitrogen the hero-like qualities that the companies trying to sell it to you would have you buy into. Yes, its dryness and purity give it a small seasonal advantage in terms of inflation stability and corrosion resistance, but these are hardly worth the additional cost and hassle of staying topped up with nitrogen versus freely available air. Nitrogen might escape your tires at a somewhat slower rate than atmospheric air, but you can’t let tire pressures ‘escape’ your attention regardless of what they are filled with if you want to drive as efficiency and safely as possible. A green cap on your valve stem is, unfortunately, no substitute for vigilance.