We just bought new Michelin tires for my Subaru Outback and the air caps are bright green. My husband says it’s because they are filled with Helium. Is this so? And if so how do we add air to low tires? Or is my husband full of hot air?


The tires are filled with nitrogen, not helium. Does the vehicle have a Tire Pressure Monitoring System? If so, that’s why nitrogen is in the tires so moisture doesn’t damage the sensors for the TPMS. If the vehicle doesn’t have a TPMS, then you fill the tires with air.


Thank you

Yes, it was nitrogen. I hope you didn’t pay extra, it’s a ripoff. I know of no manufacturer that recommends it. Please do not let the green caps discourage you or your husband from keeping your tires correctly inflated with air. Air will do absolutely no harm.

Nitrogen, not helium.

FYI: Air is 78% nitrogen, so pure nitrogen doesn’t really make much difference.

If hubby paid extra for the nitrogen he wasted his money.

Nitrogen contains no moisture. Compressed air contains moisture. This moisture is what causes damage to wheels, and cause problems with TPMS valve stems.


Tester, while I have great respect for you and cannot disagree with your post that “house air” conatins moisture that pure nitrogen does not, I’m unfamiliar with a case of moisture in “house air” damaging a pressure sensor.

I’m curious, has anybody out there seen a case of this? My enquiring mind wants to know.

I know the new GT-R comes from the factory with nitrogen filled tires, so I’m guessing they’ll recommend it.

Maybe, but the tire makers don’t. Will nitrogen hurt? Absolutely not. Is it worth anything extra? Not in my opinion.

I haven’t heard of moisture causing problems with TPMS sensors, but I know the compressed air I pump into my tires contains moisture, and I suppose it could be problematic.

This is the ONLY good reason I’ve yet heard for the use of nitrogen in tires, and I’m willing to agree that perhaps, with in-tire sensors, nitrogen is a good idea.

Perhaps, but as we’ve discussed before the “house air” that was in the tires when they were mounted in the rims is still there along with its moisture. Pure nitrogen is just added to bring it up to pressure. The “house air” was never removed. Unless, of course, you have one of those fancy two-valve rims and purge it.