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Yep. So the tire acts a molecular sieve. The oxygen is more likely to leak out than the nitrogen. The concentration of nitrogen gradually increases.
Agree. The tire is like a nitrogen generator. Over time all the "bad air" will leak out leaving just the stuff (like nitrogen) that leaks more slowly.
 

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I do remember back when I was running 38 psi air in my '07 TCH tires. I rented a tank of nitrogen as nobody in town provided a nitrogen tire service.

One night I filled the tires to 37 psi cold with nitrogen. I went for a some test drives at 40 to 60 mph including some swerving and found the tires felt sluggish, like the tires had low air pressure. I notice a place were I can coast for 1/8th mile. Though that area the tires didn't let the car coast free as it did before.

I allowed the tires to cool again then aired them up to 40 psi later that night. I took the car out again to drive again on the highway. The steering was crisp again. I could tell the difference in the extra 3 psi. Even at 40 psi nitrogen, the then michelin energy saver tires rode smooth with the '07 TCH.
 

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Yep. So the tire acts a molecular sieve. The oxygen is more likely to leak out than the nitrogen. The concentration of nitrogen gradually increases.
Talking about leaking air out of the tires. That reminds me of the many brands of new tires I had bought over the years. I remember way back some brands you had to add air every few weeks. Other brands would go near a month and need 2 or 3 pounds of air.

I did find after switching to Goodyear tires (in the early 80's) then to Michelin's, they would go for a few months before needed any air. This could be another reason the nitrogen I put in the '07 TCH went for a full year before having to add 2 pound in each tire to bring them back up to 40 psi. This may been the higher grade of nitrogen as it was from a welding supply. The car dealer put in the lower grade of nitrogen in my '12 TCH.
 

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Agree. The tire is like a nitrogen generator. Over time all the "bad air" will leak out leaving just the stuff (like nitrogen) that leaks more slowly.
So this is great news. I can have my cake and eat it too. Put free air in using my home compressor and I will have mostly Nitrogen in the tires over time as the O2 leaks out.
 

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WhiteSands,
What would be the difference between low grade Nitrogen and high grade Nitrogen?
Curtis_TX answered that basically here talking about nitrogen vs oxygen. If so, one would think that nitrogen, N78% would leak out a little more than N98%.

The tire acts a molecular sieve. The oxygen is more likely to leak out than the nitrogen. The concentration of nitrogen gradually increases.
 

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I recall that Consumer Reports claimed that most tire shops use 95% nitrogen. However, the amount you get in your tires is going to depend heavily on how many times the tire guy purges the tire. You can't totally remove the air from the tire unless you draw a vacuum on it. So the reality is that at 0 psi gauge, you have 15 psi absolute still in the tire. If you inflate to 45 psi gauge that is about 60 psi absolute. In other words on the first fill you have only displaced about 75% of the air. Fortunately that air is 78% nitrogen. If they do two or three purges then you will start to reach the % nitrogen in the supply they use. I'm guessing the typical tire guy (or gal) only does one fill.

At the same air pressure nitrogen is going to do nothing for gas mileage though. The only benefit it has if you are lax in keeping the pressure up, it will leak off a bit slower, and that is rather trivial.
 

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I recall that Consumer Reports claimed that most tire shops use 95% nitrogen. However, the amount you get in your tires is going to depend heavily on how many times the tire guy purges the tire. You can't totally remove the air from the tire unless you draw a vacuum on it. So the reality is that at 0 psi gauge, you have 15 psi absolute still in the tire. If you inflate to 45 psi gauge that is about 60 psi absolute. In other words on the first fill you have only displaced about 75% of the air. Fortunately that air is 78% nitrogen. If they do two or three purges then you will start to reach the % nitrogen in the supply they use. I'm guessing the typical tire guy (or gal) only does one fill.

At the same air pressure nitrogen is going to do nothing for gas mileage though. The only benefit it has if you are lax in keeping the pressure up, it will leak off a bit slower, and that is rather trivial.
You said, I'm guessing the typical tire guy (or gal) only does one fill. That reminds me of when they brought in the women to do the men's jobs. We had to learn these new terms, remove the work man and start using, telephone worker or post person.

Many of the new dealers nitrogen machines do purge the tires. They can connect the hose and go do whatever, come back later and the tire has purged maybe 5 times then filled to the desired pressure with nitrogen. I don't know if they connect up 4 hoses or do it one tire at a time. I will ask when I go for any more nitrogen which I hope it's a long time from now.

The idea of nitrogen helping with the gas mileage, is by not having to add air to the tires for maybe a year. I plan on adding nitrogen as usual before winter then letting them down some if needed before the hot summer. I may run 42 this winter as we have no ice and is very rare to ever see snow here. That way when I drive off on a very cold day I will be riding on 42 psi tires that won't heat up at all when driving to town.

Messing around yesterday I got up to 60 mph for five minutes, looking at the nav display, the 40 psi tires then read 42 psi due to the hot pavement. The outside temp was 95 degrees.
 

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The idea of nitrogen helping with the gas mileage, is by not having to add air to the tires for maybe a year. I
That doesn't help with the gas mileage. It's just more convenient.

If for example the tires are pumped up whenever the pressure is down 2 PSI it doesn't matter whether it's done annually or every 6 months or even every month. The average PSI is -1 PSI from ideal for the year any way it's done. Nitrogen doesn't help gas mileage a bit. It just saves a little effort.
 

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That doesn't help with the gas mileage. It's just more convenient.

If for example the tires are pumped up whenever the pressure is down 2 PSI it doesn't matter whether it's done annually or every 6 months or even every month. The average PSI is -1 PSI from ideal for the year any way it's done. Nitrogen doesn't help gas mileage a bit. It just saves a little effort.
I'm simply repeating what I have read on the benefits of using nitrogen instead of air in the tires. Their must be some benefits due to the cost of the nitrogen equipment machines being sold to many new car dealers. Could be the simple reason to help make the car more maintenance free. They have done this with the spark plugs, solid state ignition, lifetime coolant and etc.
 

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I'm simply repeating what I have read on the benefits of using nitrogen instead of air in the tires. Their must be some benefits due to the cost of the nitrogen equipment machines being sold to many new car dealers. Could be the simple reason to help make the car more maintenance free. They have done this with the spark plugs, solid state ignition, lifetime coolant and etc.
The only benefit I can attribute to nitrogen is that it leaks more slowly. So, if you reset your tire pressure on a fixed time schedule the pressure will drop less between adjustments. And, if you assume there is a fuel mileage drop associated with the lower pressure, then there is a very slight loss with using air. 13 cents worth of fuel per year, by my calculation. However if you set your tire pressure twice as often when using air only, then there would be no savings with nitrogen.

I think the short story on nitrogen is that they have taken that one very marginal benefit and over promoted the use of it.
 

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Its 8:40 pm, no hot sun for a couple of hours. Just used my new gauge to put 39 psi in all 4 tires. TPMS however shows 40 in all tires. I ofcourse waited for TPMS to catchup but still show 40 after 5 mins.

So beware! Your TPMS could be off. Mine is off by 1 PSI over and above it's actual PSI.

Cheers.
 

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1 PSI won't have any effect on anything. It sure won't hurt anything. You never know if TPMS or the gauge is the one that is off. Nothing is 100% accurate.
 

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OK...

Well beat yourself up over thinking you are 1 pound off then. I doubt the accuracy of the TPMS and gauge are anywhere near that perfect... but believe as you like.
 
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