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. But this was kind of a last-minute decision, and I confess 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, since not all garages are equipped with nitrogen: (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? (I dread the expense!) 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. I'll look forward to your answer!

-- Clem

Car Talk: Great questions, Clem. And the answer, as my brother would have said, is: fuggedaboutit. 

I think 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. You bit on the pinstripes, the pom-pom on the antenna in the shape of Vladimir Putin and the nitrogen-filled tires. And all that only added $30 to your monthly payment! Congratulations.

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. And then some salesmen have claimed that if you have an accident and there's a fire, if your tires explode there won't be additional oxygen to further fuel the flames. 

Every one of these arguments has an iota of truth. But they're all completely overblown. And 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! And 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. Tires in good condition don't leak. 

Fourth, a small variation in tire pressure while driving is normal. Pressure increases as the 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. 

And finally, if your car is already on fire, to the point that it causes your tires to heat up and explode, a slightly brighter flame around the wheel wells will be the least of your worries. 

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. 

And no, it would not be practical to buy four new, air-filled tires. Your tires are fine, and there's no need to replace them just to change what they're filled with. As they need air, fill them with air from any convenient gas station. And over time, try to forget that you ever heard the word "nitrogen."

Good luck, Clem.