Question: I recently bought two new tires at Costco, and they had green valve caps. I thought it was just some marketing thing until I went in for service and they said they couldn’t top off my tires because they were filled with nitrogen.
What’s the deal with that? Does it hurt to add regular air? — David, Tacoma
Answer: Nitrogen contains less water vapor than regular compressed air and, because nitrogen molecules are bigger than oxygen molecules, it takes longer for them to seep through the porous walls of your tires.
Tires filled with nitrogen maintain their pressure longer – which means, theoretically, you should get better mileage and less wear.
It won’t hurt to top off with regular compressed air, but you’ll be diluting whatever advantage the nitrogen might have been giving you.
rob.carson@thenewstribune.com253-597-8693
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