Should You Fill Your Car's Tires With Nitrogen?

The Green Cap Could Take Green From Your Wallet


A member of the Edmunds forums was negotiating for a new truck at the dealership. He was presented with a price breakdown, which included a number of dealer add-ons. He'd seen most of the items before (window etching, wheel locks, etc.), but a $249 charge for a "nitrogen upgrade" caught his attention. The air in the tires of the vehicle had been replaced with nitrogen and the valve stems topped with a telltale green cap. He was able to get the fee waived, but not without a fair bit of haggling.

This isn't an isolated case. It has become increasingly common for car dealerships to charge this fee for nitrogen-filled tires on new vehicles. The prices can range from $100 as a stand-alone item to more than $700 if it is bundled with other items such as window tinting or door edge guards.

Nitrogen proponents claim that filling your tires with this gas will save you money on fuel, prevent wheel rot and offer better performance than old-fashioned air can give you. But a closer look reveals that nitrogen has few benefits and much higher costs. Here's the short version: Save your money and stick with air.

Why Is Nitrogen Good for Tires?

The Get Nitrogen Institute, a nonprofit group that promotes nitrogen, says that with nitrogen tire inflation, drivers will note improvements in a vehicle's handling, fuel efficiency and tire life. All these benefits are achieved through better tire-pressure retention and cooler-running tire temperatures, the institute says.

It sounds great in theory, but let's take a closer look at each of those claims:

Better tire-pressure retention: Over time, a tire will gradually lose pressure. Changes in temperature accelerate this loss. The rule of thumb is a loss of 1 psi for every 10-degree rise or fall in temperature. The institute says that nitrogen has a more stable pressure than oxygen since its larger molecules are less likely to seep through the permeable tire walls. But what does that mean in the real world?

Consumer Reports conducted a yearlong study to determine how much pressure was lost in tires filled with nitrogen versus those filled with air. The results showed that nitrogen did reduce pressure loss over time, but it was only a 1.3 psi difference from air-filled tires. Among 31 pairs of tires, the average loss of air-filled tires was 3.5 psi from the initial 30 psi setting. Nitrogen-filled tires lost an average of 2.2 psi from the initial setting. Nitrogen won the test but not by a significant margin.

Improved fuel economy: The Environmental Protection Agency says that underinflated tires can lower gas mileage by 0.3 percent for every 1 psi drop in pressure of all four tires. The theory is that since nitrogen loses pressure at a slower rate than air, you are more likely to be at the correct psi and therefore get better fuel economy.

If you are proactive and check your tire pressure at least once a month, you can offset this difference with free air, and you won't need expensive nitrogen. We think this invalidates the "better fuel economy with nitrogen" argument.

For many people, however, this kind of maintenance is easier said than done. Most forget to check and top off their tires regularly or they never learned how to do it in the first place.

And though tire-pressure monitoring systems (TPMS) now come standard on cars, a National Highway Transportation Safety Administration study found that only 57 percent of vehicles with TPMS had the correct tire pressure. That's because most systems are only meant to signal that a tire has very low pressure, not to show that the pressure is optimal. Presumably, nitrogen-filled tires would save us from our own laziness, but at a price.

Cooler running temperatures: When air is pressurized, the humidity in it condenses to a liquid and collects in the air storage tank you use at the local gas station. When you add compressed air to the tire, the water comes along for the ride. As the tire heats up during driving, that water changes to a gas, which then expands, increasing tire pressure. Because nitrogen is dry, there is no water vapor in the tire to contribute to pressure fluctuations.

But this fluctuation in temperature isn't as significant as you might think. An ExxonMobil study plotted the changes in temperature over the course of various inflation pressures. The lines on the graph were virtually on top of each other. In other words, the change in temperature when using nitrogen was negligible.

Wheel rot prevention: Nitrogen proponents argue that water in a tire can lead to wheel rot. A tire engineer who anonymously maintains Barry's Tire Tech, a blog on a number of tire issues, says this isn't really a problem with modern cars.

"Alloy wheels don't really have a problem with water inside the tire," the engineer writes in a post on nitrogen inflation. "They are coated to keep aluminum from forming aluminum oxide, which forms a crust, which isn't very attractive. But even then, this crust protects the aluminum from further corrosion from the water."

Where wheels have problems is when the aluminum alloy contacts steel, such as the steel spring clip used on wheel weights. It's a particular issue when salt is present, the engineer writes. "But this problem is totally independent of the inflation gas," he says. "Steel wheels only have a problem if the paint is damaged."

Maintenance cost and convenience: Dealer costs aside, there are also maintenance costs to consider if you switch to nitrogen. Let's say you bought a set of tires at Costco, which uses nitrogen to fill all the tires it sells. If you need to top off the tires with more nitrogen, you can't go to just any gas station. Granted, you can use regular air if nothing else is available, but that would dilute the nitrogen in the tires. You'll have to go back to the shop with nitrogen and wait until the tire technicians can attend to the car. On a busy day, you could be there a while.

Nitrogen is free at Costco and at some car dealerships we called, but these are rare cases. We called a number of tire shops that carry nitrogen and found that the prices for a nitrogen fill ranged from $7 to $10 per tire. Assuming you're diligent about checking your tires monthly but can't make use of a free nitrogen service, you could potentially spend a few hundred dollars a year on nitrogen. Compare that to most gas stations where air is free or $1.50 at the most for a fill-up of all four tires.

Is Nitrogen Worth It?

The air we breathe is made up of 78 percent nitrogen, 21 percent oxygen and a few other elements. In order to get the desired benefits for tires, the nitrogen used needs to be at least 93 percent pure, according to TireRack.com. So we're essentially talking about adding an extra 15 percent of nitrogen and getting rid of as much oxygen as possible.

Based on cost, convenience and actual performance benefit, we don't think nitrogen is worth it. A much better use of your money would be to buy a good tire-pressure gauge and check your tires frequently. Having the correct tire pressure will get you many of the benefits of using nitrogen and will ensure that your tires last longer.