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Why do some Tires have Green Caps?

Tires with green caps are filled with nitrogen.
Tires.
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  • Written By: Carol Francois
  • Edited By: Bronwyn Harris
  • Last Modified Date: 19 August 2014
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Tires with green caps are filled with nitrogen, instead of compressed air. Nitrogen is a naturally occurring, colorless, odorless, tasteless gas that makes up 78% of the earth's atmosphere. The vast majority of tires are filled with compressed air. Nitrogen-filled tires are a recent development and are proving popular with consumers in climates that have large shifts in temperature during the course of the year.

Nitrogen-filled tires have green caps so that everyone will be able to recognize that they are not filled with air. There are two major benefits to these tires: they reduce the rate of gas loss and they eliminate moisture from within the tire. Nitrogen-filled tires are slightly more expensive than standard tires due to the cost of nitrogen and the benefits the tire provides.

Tires filled with nitrogen retain the optimal pressure levels for longer than air filled tires. This creates a more even wear pattern in the tire and provides better gas mileage. Nitrogen molecules are fatter in shape than oxygen molecules, and as a result, they have a lower leakage rate through the tire walls. These walls are porous in order to allow the rubber to expand and shrink as needed.

Humidity is the level of moisture in the air. Moisture is held in oxygen molecules, so a tire with no oxygen, such as a nitrogen-filled tire, is not affected by humidity. This difference results in a more consistent tire pressure.

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The fastest way to improve gas mileage and extend tire life is to ensure that tire pressure is checked regularly and is at the appropriate levels. The National Highway and Traffic Safety Associations (NHTSA) reports than less than 60% of American motorists regularly check their air pressure. Nitrogen-filled tires retain their pressure level for longer, providing a lower maintenance product than standard tires.

Over time, the nitrogen levels will decrease, and more nitrogen will need to be added. Drivers can contact their local automotive or tire dealership to fill the tires with the correct gas. Filling the tire is usually a fee based service, with a flat rate per tire. The air pump nozzle is a different shape than the nitrogen pump. This is to ensure that tires with green caps are not given oxygen in error.

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Discuss this Article

anon322833
Post 9

Think of nitrogen like this. The air we breathe naturally is 78 percent nitrogen. So is 20 percent O2 in your tires really going to matter? This is just the latest scam in the auto world.

anon322832
Post 8

So after mounting the tires on the rims in a room full of regular air, how'd they get the regular air out to fill the tires with just nitrogen?

anon310472
Post 7

Sounds like a good $8 x 4 plus the spare $40 scam to me. What is the cost to change a flat? Way to go, rip-off dealerships. They also charge extra to add nitrogen, like $2 minimum per tire. Don't you just love it?

anon285691
Post 6

We use it in our service vehicles that carry heavy loads since Tire Warehouse told us that by doing so the tires will not heat up when driving on the highway as they would with air in them. Therefore, the tires will last longer. It's helped extend the life of our tires.

anon281818
Post 5

I went to a tire store to get new tires. All four on the car were nitrogen filled. They told me the green stems meant the tires had "TPMS defeaters" - that is what the green stems were for, and that I needed to have the TPMS rebuilds done for each tire at an extra cost of $8 per tire! Didn't figure it out until after I paid for my tires. What a scam!

anon151875
Post 4

If I have the green caps and my tire needs air does it hurt to put regular air in it? Regular air does have 78 percent nitrogen.

anon129751
Post 3

I just recently purchased a Nissan Maxima. It has green valve caps. No one at the dealership mentioned why and I thought maybe they just came with this brand of tire. They are a lower profile type of tire.

I went to have my 1st oil change today and the manager asked his employee who did the oil change if he had checked the nitrogen pressure in the tires. He did and they needed three lbs of nitrogen added to each tire because they were all equally at 32. I was totally clueless. I asked about them and he explained it all to me. I am so glad he did.

I told him that I bought the vehicle from another, much colder state where my husband had been working and when I got the car home to TX and the weather changed the low tire pressure light had come on. He informed me there should be 35 lbs. of nitrogen pressure in my tires. He added it and my car feels like it is driving smoother too. Glad to know there are some car guys out there who know what's going on!

anon88632
Post 2

sounds like you need a better dealership.

anon40454
Post 1

I recently purchased a Chevy Equinox and it came with green valve stem caps. I asked the dealer about whether the tires were filled with nitrogen and he said he had no idea. The service department didn't know either but tried to find out and couldn't. The dealership also doesn't have the capability to fill the tires with nitrogen should I need service.

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