green tire valve caps

posted by misterhaan in cars, scams on nov 18, 2008

some days when i get to work i end up parking next to a car that has green caps on the tire valves. i probably wouldn’t have thought much of it, but my mom told me how she has her tires filled with nitrogen instead of air. nitrogen and the color green don’t necessarily go together, but it turns out green caps on tire valves indicates that the tires are filled with nitrogen. they do this to help make sure somebody doesn’t refill your tires with air when you’ve paid extra for nitrogen.

when my mom first told me she did this, i thought it was a little silly since as far as i could remember, air contains more nitrogen than any other gas. i was pretty sure air vs. nitrogen came down to less than 100% nitrogen (i thought around 40%) vs. 100% nitrogen. could the other gasses in air really make it noticeably worse than 100% nitrogen would be?

so why would my mom (and some unknown person i work with) want to do this? like i said, i heard about filling tires with 100% nitrogen from my mom. she had been told nitrogen leaks out of tires slower than air and also expands and contracts less with temperature change. so with 100% nitrogen, your tires are supposedly at the ideal pressure for longer which means better gas mileage and slower tread wear as well as less frequent need to refill the tires.

to make sure my memory of high-school chemistry wasn’t failing me, i ran a search or two on the internet. the first thing i noticed was i was wrong about air being 40% nitrogen — it’s actually nearly 80% nitrogen! so putting nitrogen in your tires instead of air is 100% nitrogen instead of 80% nitrogen. just from that i can’t see how you’d get any noticeable benefit. turns out anybody who’s compared gas mileage and / or tread wear for air vs. nitrogen did not find any statistically significant difference. i guess the biggest difference is not needing to have your tires refilled as frequently, but since i get that done free whenever i get my oil changed i don’t really consider that an advantage. so you’re paying extra money for essentially no reason (unless you really wanted new valve caps).

filling tires with nitrogen has actually been used as a bit of a scam, and probably still is. what people are getting scammed on is being convinced it’s worth paying extra for nitrogen. my mom and probably the person whose car i sometimes park next to at work were convinced of that. sometimes the cost to get air-filled tires emptied and refilled with nitrogen is over $100 per tire, and then there’s a cost to get them topped off again later since nitrogen still leaks out of your tires, even if it is slower. thankfully my mom didn’t pay nearly that much, and was promised free nitrogen refills any time she needs it, as long as she goes to the place she paid to switch her tires to nitrogen in the first place.

i’m getting new wheels and tires for my car soon, and while i considered going with nitrogen for a little while, it didn’t take me long to decide it’s not worth any extra money to me at all.