Green caps on tire valve stems usually mean the tires are filled with
nitrogen instead of ordinary air. Car dealers, tire dealers and repair
shops have touted nitrogen for several years as a better alternative to
air based on claims that nitrogen doesn't leak as much, so tires stay
fully inflated longer. The catch is that topping off the nitrogen
typically costs from about $5 to $10 per tire, and the initial charge
for filling the tires can be much higher, so it's also a revenue source
for service outlets.
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