Why can’t I use carbon dioxide gas to fill my tires instead of air (78 percent nitrogen) or instead of 100 percent nitrogen? If EVERYONE in the world filled their tires with carbon dioxide, wouldn’t that take a LOT of carbon dioxide out of the atmosphere and slow global warming? Think of the amounts involved! Thank you!

It’s thinking like that that’s kept you out of the country’s best institutions of higher learning!

It’s actually a great theory. So let’s do the math on it.

The average car tire holds about 1,700 cubic inches of air. So if we take five tires per car (including the spare), the average car could hold about 8,500 cubic inches of carbon dioxide. If we convert that to pounds, it’s about 0.6 pounds.

But, as you say, think of the amounts involved. Let’s say there are roughly a billion cars on the road worldwide. So now you’ve sequestered 600 million pounds of carbon dioxide! Wow!

But here’s the problem: Every time you burn a gallon of gasoline, you create 20 pounds of carbon dioxide. So filling up your tires with carbon dioxide would negate the carbon dioxide produced by using 0.03 gallons of gasoline — or enough to let you back out of your driveway and run over your kid’s Big Wheel.

So, unfortunately, it’s only a drop in the proverbial carbon bucket.

And if you look at it globally, and include the carbon dioxide created by fossil fuels, power plants and cement production, we humanoids create about 60 trillion pounds of carbon dioxide a year — give or take 20 trillion pounds. And if you compare that to the meager 600 million pounds you could hide in all of the world’s tires, you’ll see that you’re not making a lot of headway.

And we haven’t even figured in the cost of capturing the carbon dioxide and delivering it to 5 billion tires.

So it’s a great concept. And we encourage you to keep thinking. But if the goal is to reduce carbon dioxide, the money and effort would be better spent replacing older fossil-fuel plants with solar or wind generation. Or figuring out how to make every factory on the planet run on cow farts.

I think our minivan might be haunted! My husband and I own a 2002 Ford Windstar. It has 97,000 miles on it. About two weeks ago, a strange phenomenon occurred: It started knocking! It started quietly, and then got louder and louder. I was quite alarmed when this began happening. I was pulling out of our driveway, with my daughter in tow. I yelled to my husband in the backyard to come quickly, explaining that something in the van was knocking. My husband strolled over and — guess what? The knocking had stopped! He said, “I don’t hear anything!” Fortunately, my daughter was able to verify the noise, or he probably would have had me committed! In subsequent days, the knocking has continued to occur at random times. I haven’t been able to establish any kind of pattern with it — it just abruptly starts and stops. The noise seems to be emanating from behind the glove box. Sometimes it is very loud, and other times it is more like a tapping sound. Usually it starts out softly and crescendos. I strongly suspect that we have a poltergeist. I guess my question for you guys boils down to this: Should we call a mechanic, or an exorcist? Your input will be greatly appreciated!

I’d go for both. Cover all the bases.

It sounds like you have a stripped gear in your ventilation system. Inside the air ducts that run behind your dashboard, there are flaps — called blend doors. They change the direction of the air (to the windshield or to your feet, for instance) and regulate how much hot air enters the cabin.

Each blend door is controlled by a small, vacuum-operated motor. That motor (really, an actuator) turns a plastic gear, and the gear opens or closes the little door to which it’s attached.

I’m guessing that one of your gears is stripped, so as it tries to move the blend door, it’s able to move it part of the way, but then the door snaps back. That’s what creates the snapping or knocking sound.

Next time you hear it, try playing around with the ventilation controls. Try moving the knobs that control the amount of heat and the direction of the air. My guess is you’ll be able to change the noise, or even get it to stop. Of course, you may find out that in order to avoid the noise, you have to set the heat at full blast directly into your face all summer. So your other option is to get it fixed. If you’re really lucky, your mechanic might be able to reach the offending blend door by removing the glove box. But if he has to remove the entire dashboard, that’s going to get expensive. Like, hundreds of dollars.

So when you find out what it costs, you may just decide to live with your poltergeist. Maybe you and your daughter can use the steady beat of the blend door to practice your rap.

Got a question about cars? Write to Car Talk in care of this newspaper, or email by visiting the Car Talk website at www.cartalk.com.