If you’ve ever noticed green-capped stems on tires in a parking lot and wondered why the car’s owner chose them, you’re not alone. It’s a popular question, and one many people answer incorrectly. Green-valve-stem caps aren’t a fashion choice — they signify that the tire contains nitrogen instead of regular air. There are benefits to this, but it’s not always easy to find a place to get nitrogen refills — and it costs more. If your car comes with
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