Hey, gang.
Have you come across tires with green valve stem caps yet? If you do, you should know it to mean that those tires have been filled with nitrogen, not compressed air.
Why would someone fill their tires with nitrogen anyhow? The biggest reason is that a tire filled with nitrogen will leak at a much slower rate than a tire filled with compressed air. If you check your tire pressure regularly and refill your tires with air, there's really no benefit. It's definitely not worth the $10 per tire some places charge to remove the compressed air and refill with nitrogen.
Most people don't maintain their cars as they should, so they end up with low air pressure over time. Low air pressure results in greater wear to a tire (tread separation in some cases) and lower fuel efficiency. Specifically, tires that are inflated to 32psi get about 3% better mileage than a car whose tires are only inflated to 24psi.
Other reasons to use nitrogen instead of compressed air are listed here, but really, these points are meaningless.
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A common myth is that you should never add compressed air to a tire that has been filled with nitrogen. Please note that topping off a low tire with compressed air won't hurt, tire experts say, and the tire can be purged and refilled with nitrogen later if they choose.