Hi Midwest,
Didn't know you lived in Minnesota. I was in Minneapolis in January a few years ago and thought that I would die. I've got some friends in the Minnesota Orchestra and they tell me stories about having to use electric block heaters in their cars. Wow. I lived at Interlochen, Michigan for several years, but never got nearly as bleak and cold as Minneapolis. But since I do drive back up to northern Michigan several times a year, perhaps your suggestion about nitrogen is food for thought. I drove up there last February when it was 65 here in DC and 20 at Interlochen, and my nitrogen filled Mercedes tires only changed by 1 psi. But with run flats, maybe this is all moot?

I think that's the key -- we have RFT's...I love the change of seasons, but winters here are too long..Am ready for Spring by end of Feb :cool:

Sorry I forgot to look at your X6 specs and saw that you have the 20" OEM performance run flats. I was told by my PA dealer that if I plan to drive up north a lot in the winter to go for the 19" all season with the sports package. Maybe he was wrong. My hat is off to you for dealing with the bitterly cold weather in Minnesota. Beautiful in the summer, but it does take a strong constitution to deal with that winter. Garrison Keillor loves to make jokes about the Minnesota winters in his books and radio show. I thought he was exaggerating, but then I had the "winter experience". Do you go ice fishing" I did it once when I taught up in Michigan, and somehow managed to fall in. Students were laughing their heads off. To add insult to injury, I only caught one three inch fish after standing out on the lake for two hours before putting my right leg into an open hole in the ice that had been snowed over. Best of luck to you with your amazing X6 (it is beautiful), and you do a fantastic job if keeping it clean.
It is wonderful to be back in the BMW fold. BMW people are much more interesting than the Mercedes crowd.

JB essentially answered it, but the fault here is my short/cryptic post.

To explain the physics, as I said in my post, air expands when heated just like nitrogen does. They are essentially equal in that respect. However, moist air (which includes water vapour) expands much more, comparatively speaking. It is also the moisture that causes the corrosion problems with the rim.

Racers use nitrogen, and remember that they can get tires much hotter than street use. This isn't because the nitrogen doesn't raise the pressure compared to air but rather that it does so in a much more predictable manner. Racers will measure hot pressures (they have to) and wet air will produce much more variance in hot pressures than either dry air or nitrogen. In those cases, nitrogen is just easier to use. For those of us not measuring hot pressures and bleeding tires down in between laps, but rather driving on the street, using nitrogen is nothing more than a way to get a green valve stem cap.

Thanks for the explanation. Makes a lot of sense. When I had nitrogen put into my recently retired Mercedes E class (which they did for free) I talked the dealer into not putting on those rather ugly green valve stem covers. Those things would definitely not look acceptable on any BMW.

My tire place put nitrogen in my Toyo's when I had my tires replaced a few years back. I would have never asked for it, but I have noticed that even after two years and 30,000 miles, I have only had to top off the pressures once or twice to maintain spec. Also, the tire shop that installed the tires offers free checks and top ups, so there was no extra cost.

As you said, performance benefits on the street are nill, but I do see how it could be a slight safety benefit to someone who doesn't check their air pressure as much as they should. Especially an SUV.

I had the same experience with my now retired Mercedes. Had nitrogen put into the Conti tires when car was new, and didn't have to have any top up whole two years I was driving it. But if you do have a leak, or lose some pressure somehow, you cannot top up nitrogen fill with regular air.

What happens if you do this? It just negates the point of the nitrogen, correct?

I am going to have to replace my tires in the next few months and I was actually thinking about just topping up with regualr air if the levels happen to below instead of making a trip to thetire place and waiting.

I was told by the Mercedes dealer (who may or may not be correct) that you should not top off a nitrogen filled tire with regular air. That, according to him, will introduce some water into the mix, thus negating the effectiveness of the "dry" nitrogen.

I don't buy it. Who's going to run around looking for a N2 top up when their tire is low? The important thing is to not drive around with underinflated tires, not whether or not you may have 10% regular air vs pure N2.

Introducing water is another thing. Even if you have regular air in your tires you should avoid poor quality air (moist). I've seen gas stations that had water dripping out of the air hose. :yikes: