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Better Gas Mileage with Nitrogen

Huh? From comes this:

I had the oil changed in my car over the weekend. As the guy was explaining the services they performed, checking the wipers, etc, he said "Normally we check the tire inflation but noticed that you had green valve caps. They're filled with nitrogen and since nitrogen doesn't leak out like air does, we didn't check them." Think I said "REALLY!" five times. Guess I wasn't paying attention when I bought my tires at Dunn Tire. My gas milage has increased about 7 percent. Thought it was just the new tires. Must be the combo of the tires and the fact that they're filled with nitrogen. Nitrogen filled tires are supposed to get 3-5 percent better fuel mileage.

Regular old air is ~78% nitrogen, and a quick Google search yielded very little fact. Supposedly ol' N oxidizes the inside of your tires less, leading to less wear. Then again, airlines fill their tires with nitrogen, but that may have more to do with a fire in the fold-up wheel compartment (and the trouble having a lot of fresh oxygen would cause).

But hey, some facts for you based on my background in science:

  1. Air is 78% nitrogen (N2) and 20% oxygen (O2).
  2. PV=nRT is the formula for ideal gases. Inside a tire, this law is obeyed as it's not under extreme heat or pressure.
  3. N and O are next to each other on the periodic table. They have similar specific heat capacity. Nitrogen is actually smaller and weighs less, so if anything is gonna diffuse it's nitrogen.

The weight savings, by the way, are orders of magnitude away from "3-5%," and more so when you look at the weight of the rubber in the tire (let alone the entire weight of the car).

4 Responses to "Better Gas Mileage with Nitrogen"

  1. I've heard the same thing about my parent's cars. I knew as soon as they said, "Well, the dealer said that nitrogen doesn't leak as much as air" that the mechanic knew nothing about why they were using nitrogen. Like you said, air is 78% N2, so why should purifying the other 20% mean it never needs to be filled?

    I'd be willing to bet that ErieBlogs's newfound mileage increase is from two things: 1. new tires with new treads 2. tires inflated to the proper air pressure. I have a very good idea of the highway mileage on my Integra, however (34-35 mpg), so maybe I'll go get mine filled with nitrogen and see what happens.

  2. Hello!

    I'm an aircraft mechanic, have been for 25 years. I have worked on airliners for the past 20. Just thought I'd tell you why we use nitrogen in airplane tires.

    First of all, not all A/C tires are nitro filled. I would guess that the VAST majority of general aviation category airplanes (small private A/C) have regular old shop air in the tires. Nearly all of them I have worked on have.

    There are 3 reasons I can think of that it is used on heavier/higher performance A/C:

    Primarily it is because of brake heat. A brake pack on a big bird (I presently work on DC-10's & Airbusses) generates a huge amount of heat on landing. There is a chart in the flight and maintenance manuals on some A/C to plot the amount of energy generated by a brake on a particular landing. This is variable due to the fact that the weight of an airliner can vary greatly from flight to flight, and not all runways are the same length. After a high energy stop, there is usually a mandated cooling period before another takeoff can be attempted (in case an emergency stop might be required on a subsequent takeoff, you don't want to be starting out with baked brakes). The energy generation chart is used to figure the cooling time. On some of the charts I have seen, the top of the chart is over 10 MILLION foot pounds of energy absorbed, per brake. (Yes, I know ft/lbs. is not a "proper" energy unit, but that's what the chart uses). Newer A/C just directly measure brake heat & display it in the cockpit. The brakes weigh 200-300 pounds, and it is not uncommon to see them still glowing red 30 minutes after a normal landing on a hot night. Needless to say, this puts a huge amount of heat in the rims, and thus the tires. This of course causes the pressure to rise, and there are thermal plugs in the rims to vent the tire pressure in case they get TOO hot, so they don't explode. I have seen and had the unhappy privilege to have worked on numerous A/C that have thermaled tires. I have seen brakes still glowing cherry red nearly 2 hours after landing. I had to change ALL 16 MAIN TIRES on a 747 one time due to this. I invented 3 new words that night.
    What all of this is getting at is that all tires, and particularly big tires, have a certain amount of debris & fuzz inside them; this could supposedly cause internal fires when subjected to this sort of heat if there were any oxygen available, thus the dry nitro charge.

    Second, dry nitro out of a bottle has no moisture/oil in it as shop air from a compressor could, thus potentially reducing rim corrosion/tire degradation. It is just a more reliable & clean source than compressed air.

    Third is pure logistics. Normal tire service pressure on big birds is around 200 psi. Not many normal air compressors will deliver that high pressure. Also, nitrogen is used for many other things on an airplane, most of them at much higher pressure (numerous accumulators are serviced anywhere from 500 to 3000 psi.). Obviously this could not be easily done with an air compressor, and these other components enjoy the same o2 free benefits as the tires. So, the nitro bottles have to be there anyway, and are an easy & relatively safe high pressure gas source.

    Other than the obvious benefit of being moisture free, there is only one reason to use it in car tires:

    advertising hype.

    As far as the mileage increase goes (due to the gas swap alone) - PURE BULLSHIT.
    Think about it - if the car manufacturers could get a 3-5% boost in mileage to help them meet the ever increasing Corporate Average Fuel Economy regulations simply by filling the tires with nitrogen, don't you think they'd have done it a long time ago, rather than spending cubic millions on R&D?
    I don't doubt that lots of cars get way better mileage after a nitrogen fill. This is probably because 30 psi
    of nitrogen replaced 11 psi of air. The average toad only knows what pressure is in their tires when they are flat.
    This is why all of the SUV's rolled over, in my opinion.

    That's my opinion, Chris Miller.

    over and out.

  3. Thanks Chris !! I and everyone else that reads your comments couldn't agree more.

  4. I have tried nitrogen in my tires and got noticed an improvement in my gas mileage. I keep nitrogen in all my tires;)

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