Filling one’s tires with nitrogen is one of those things, like rubbing cheetah’s blood on your engine block, that’s taken on almost mythological status among gearheads. The inert gas nitrogen is given credit for everything from improved handling to better MPG to reduced wheel wear. How much of this is true? What does filling your tires with nitrogen actually do? Let’s find out.
If you want a sample of nitrogen of your very own to scrutinize, just cup your hands right now. The air trapped inside that little flesh-bowl is 78 percent nitrogen. Yep, even when you’re just filling your tires with plain old air, you’re mostly filling your tires with nitrogen, anyway. So when we talk about the advantages of nitrogen over air, we’re really only talking about a 22 percent change in what the tires are filled with.
And that’s where all this gets interesting, because it’s really a lot less about the nitrogen itself, and more about the 22 percent of air that’s not being pumped into your tires.
Nitrogen itself is referred to as an inert gas (meaning it usually won’t react with anything) with an atomic number of seven on the periodic table. That’s one less proton than oxygen, which has an atomic number of eight. Nitrogen atoms bind together in twos to form dinitrogen, or N2, which is the form that nitrogen takes in the air.
So, what are the big advantages that nitrogen is supposed to provide? Most commonly you hear that nitrogen-filled tires don’t seep out as much gas, they provide better handling, better MPGs, and keep much more consistent pressure over a range of temperatures.
First, let’s look at the issue of gas seepage.
The kinetic diameter of a nitrogen atom is actually just a bit bigger than an oxygen atom. This means that it is a larger “target” and is therefore ever so slightly more likely to collide with another molecule. This is the basis for why many sources say that nitrogen-filled tires are less likely to lose pressure over time: the bigger molecules are less able to permeate the tire’s rubber and seep out.
Sources claim that oxygen atoms (in their O2 form) permeate rubber 3-4 times as fast as nitrogen. This doesn’t mean air-filled tires deflate 3-4 times faster, because, remember, there’s still nearly 80 percent nitrogen in air filled tires, so the 20 percent of oxygen may permeate 3-4 times faster. I’m not smart enough to know how much of a difference that means, but our own David Tracy is:
This is totally simplified, and doesn’t account for the change in pressure differential as the tire deflates, but here’s the difference in flow rate at time = zero:
The volumetric flow rate exiting the tire with air in it is
Vdot_Airtire = 0.8*vdot_N2 + 0.2*vdot_02
vdot_02 = 4*vdot_n2
So substitute that in. And you get:
Vdot_airtire = 0.8*vdot_N2 + 0.2*4*vdot_N2= 1.6*vdot_N2
So, 1.6 times.
So, there you go. An air-filled tire loses air at a rate of 1.6 times a nitrogen-filled tire. So, sure, nitrogen is better, but the difference is likely too minuscule for most drivers to notice.
Does this gas use less gas?
As far as the idea that nitrogen-filled tires will help you get better MPG, while there’s an argument that can be made, the truth is that it’s not really the nitrogen that’s making the difference. Tire pressure makes a huge difference in MPG. Having hard, high-PSI tire pressure does reduce rolling resistance and improve fuel efficiency, usually at the expense of handling.
Nitrogen-filled tires can be more consistent about PSI (more on that next), but it’s more about maintaining the optimal tire pressure as opposed to what’s doing the pressurizing inside the tire. So, if you keep your tire pressure at optimal levels, an air-filled tire should be able to be as efficient as a nitrogen-filled one.
It’s all about what’s missing.
The biggest advantages cited about nitrogen-filled tires have to do with how nitrogen-filled tires maintain a much more consistent pressure, even as the tire heats up, than air-filled tires.
As a BMW representative explained to me:
The main advantage of using Nitrogen in tires is less pressure growth as the tire heats up.
And the reason that nitrogen suffers from less pressure growth is primarily due to one thing:
Moisture in the air in tire cause more pressure rise as the tires heat up. What is worse is that the pressure increase is somewhat unpredictable since depends on the humidity of the air added to the tires.
Water. Humidity. Clamminess. The real hero here isn’t nitrogen, it’s the lack of water vapor. About 1 percent of regular air is water vapor at sea level, and when that air is compressed to go into your tire, that concentration can increase as well.
Water vapor is much more susceptible to changes in pressure from thermal changes. It expands when hot, contracts when cold, and changes the tire’s PSI as it does, which can cause problems for precision track and racing cars with handling tuned to very specific tire pressures.
Tire temperatures will increase while driving, due to friction and rubber deformation, and these temperature increases are independent of ambient air temperature. Tires can increase in temperature by around 50° over 30 minutes of highway driving (and much more for racing tires), and if you look at a chart of the vapor pressure of water at varying temperatures, you can see that a temperature increase like that can cause dramatically higher pressures.
Let’s say a tire starts at 60°F and goes up to, say 90°F; in that case, the water vapor pressure more than doubles. That’s a big deal.
Also, water can freeze, which is why nitrogen is specified for aircraft (and at least a couple spacecraft, like the Space Shuttle orbiter and that Air Force X-37B mini-shuttle).
So, really, the advantage of nitrogen is that it’s very dry, unlike air from a compressor, which may have a good bit of water vapor in it. It’s possible to dry out regular air as well, but it’s not terribly efficient and no easier or cheaper than extracting nitrogen, which, because of the processes used to isolate it, will always be free of water vapor.
Aside from keeping a more consistent pressure, there’s other advantages to removing water vapor from your tire: the water vapor can cause rust and corrosion inside your valve stem or to your wheel itself, so why deal with that crap? There’s also the idea that oxygen is more reactive with the rubber of the tire, and replacing that with a more inert gas like nitrogen will help the tire life as well.
So, what’s the deal?
Yes, nitrogen is technically a better gas to fill tires with than air, though it’s not really so much about nitrogen itself as it is not having water vapor in your tires.
Is it worth it? For normal driving, probably not. The advantages, while real, are still really very miniscule. Also, remember, we live in a big soup of air, so even if your tires are full of nitrogen—and that actually means about 93 percent 95 percent nitrogen—there’s still a whole atmosphere of damp, oxygen-tainted air pushing its way into the tires.
If you’re a professional racing driver or even just like to track your car a lot, then, sure, nitrogen-filled tires may give you just that little bit of edge for handling and will keep their pressure better longer-term which may give a slight edge on the track. It can’t hurt, certainly.
Also, unlike oxygen, nitrogen does not support combustion, so there’s a bit of a fire safety advantage as well.
If you have the money and can tell when one of your tires is 1 PSI less than the rest, or you have a pit crew and make a living racing cars then, please, jam as much nitrogen into your tires as you want.
For everyone else, I think you can get by just fine by bragging that you fill your tires up with a special mix of 78 percent nitrogen.