Green caps on tire valve stems usually mean the tires are filled with nitrogen instead of ordinary air. Car dealers, tire dealers, repair shops and even some gas stations have touted nitrogen for several years as a better alternative to air based on claims that nitrogen doesn’t leak as much, so tires stay fully inflated longer. What’s the truth behind nitrogen?
Related: How to Properly Check and Fill Tires
Proponents also claim that nitrogen improves fuel economy and allows vehicles to get better gas mileage than those with tires that are filled with air. With gas prices skyrocketing, the improved fuel efficiency alone may make you consider switching to nitrogen. The catch is that topping off the nitrogen typically costs from about $5 to $10 per tire, and the initial charge for filling the tires can be much higher, so it’s also a revenue source for service outlets. Does your vehicle really benefit when you opt for nitrogen instead of air, or is it just a way for service stations to make more money? Let’s take a closer look at the benefits of filling the tires on your vehicle with nitrogen.
Benefits of Nitrogen-Filled Tires
It is true that pure nitrogen doesn’t leak out of tires as quickly as ordinary air, which contains oxygen, simply because a single molecule of nitrogen is larger than an oxygen molecule, so nitrogen-filled tires should remain at or near their recommended inflation levels, or PSI, longer. Fully inflated tires last longer and improve fuel economy and road-holding ability. It’s also true, however, that nitrogen will still leak out of tires over time, just not at the same pace as air. As a result, even if you use nitrogen, you will still need to stop and check your tire pressure from time to time to make sure they are still at the correct PSI. It’s also true that a nitrogen-filled tire isn’t 100 percent pure; about 93 to 95 percent of what’s inside is nitrogen, but the rest is air.
Nitrogen proponents also argue that air contains moisture, which can build up inside tires and possibly corrode the wheels and tire pressure monitoring systems (which track your PSI and alert you to any problems), as well as accelerate tire rot from the inside. Some air compressors use dryers that remove moisture before it gets into the hose you use to fill your tires, but many don’t. Nitrogen, in comparison, provides a “dryer” inflation. The absence of moisture in dry nitrogen ensures a more steady temperature inside the tire. This is why nitrogen tires are common on race cars.
On the other hand, as mentioned, a nitrogen-filled tire is only 93 to 95 percent nitrogen. Ordinary air is 78 percent nitrogen, so the difference isn’t huge, and compressed air is cheaper to use and still free in some places.
In addition, whether a tire is filled with ordinary air or nitrogen, it will leak if the tire valve is faulty, if the tire isn’t properly mounted and sealed on the wheel, or if the tire has a nail in the tread or other damage. Nitrogen doesn’t fix damaged tires, so it’s still important to check your tire pressure regularly. Temperature changes may also still cause fluctuations in tire pressure even when you use nitrogen instead of ordinary air.
The Bottom Line
The Rubber Manufacturers Association, a trade group for tire manufacturers, says nitrogen “may contribute to minor reductions in inflation pressure loss,” but also notes that “use of nitrogen alone is not a replacement for regular inflation pressure maintenance.” Tire maintenance is important whether you opt for nitrogen tire inflation or you prefer to use air. While you may be able to go longer between needing to add air, you still need to check your tire pressure regularly.
Editor’s note: This story was updated March 2, 2021, to delineate between pure oxygen and ordinary air, which is minority oxygen.
Cars.com’s Editorial department is your source for automotive news and reviews. In line with Cars.com’s long-standing ethics policy, editors and reviewers don’t accept gifts or free trips from automakers. The Editorial department is independent of Cars.com’s advertising, sales and sponsored content departments.