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Car Talk: Tires don’t care what they’re filled with

Dear Car Talk: My questions involve the nitrogen-filled tires on my 2011 Subaru Outback. These were on the car when I bought it, and I was assured that they had advantages over air-filled tires. I didn’t do any research into the matter. My husband and I are snowbirds, and recently, just before leaving for Florida, I had a flat tire. This brought questions to mind: (1) Is it OK to drive a car with one air-filled tire and three nitrogen-filled tires, as suggested by the fellow who changed the tire? (2) Can existing nitrogen-filled tires be refilled with air? (3) Would it be more practical to just bite the bullet and buy four new air-filled tires? My main concern is driving between North and South on the interstates and getting a flat, in the middle of nowhere, and not having access to a nitrogen supply. – Clem

A: The nitrogen-filled-tire thing is a scam. Obviously, as you were in the final hours of purchasing your new car, they sold you on a bunch of add-ons. The argument they make is that the nitrogen doesn’t contain oxygen, like normal air does. That’s said to give you several advantages if you put it in your tires:

They say the oxygen in the air degrades the rubber more than nitrogen does. They say nitrogen molecules are slightly larger than oxygen molecules in the air, so it’s less likely to leak through the rubber. They say if you use pure nitrogen from a tank, there’s no water vapor in it, like there is in the air we breathe. Water vapor can affect pressure variation when the tires heat up, and can cause corrosion.

Every one of these arguments has an iota of truth. But they’re all completely overblown. In my opinion, it’s a total waste of money to put nitrogen in your tires.

First of all, air is already 80 percent nitrogen. So even if you go to the gas station and fill your tires at the ding-ding pump, you’re getting mostly nitrogen anyway.

Second, even if nitrogen limits the degradation of the rubber, your nitrogen-filled tires are still exposed to air on the outside of the tires. Once the outsides of the tires degrade, you have to replace them.

Third, nitrogen molecules are slightly larger than oxygen molecules, but it’s not like we’re having an epidemic of air seeping through rubber tires in this country.

Fourth, a small variation in tire pressure while driving is normal. Pressure increases as tires heat up. If you’re driving a race car at 200 mph, half a pound of tire pressure might make a difference, but you and I will never notice it.

Fifth, before you spend a lot of money preventing water vapor in the air inside your tires from hastening the corrosion of your wheels, remember that the other 99.5 percent of your car is always exposed to air – and water vapor.

So to answer your specific questions: It’s fine to drive a car with three tires filled with nitrogen and one tire filled with air (80 percent nitrogen); it makes no difference whatsoever.

Existing nitrogen-filled tires can be refilled with air at any time, to any degree. The tires don’t care what’s in them.