While they usually go unnoticed until you lose one, tire valve caps do serve an important purpose. But are they universal, too? Here's what you need to know.

Anyway, when you see a green cap on the air valve of a tire that means those tires take nitrogen versus regular, 'ol air according to the Family Handyman website.

Updated Gas-Mileage Aid: Valve Caps That Change Color With Tire Pressure Stephen Edelstein November 8, 2013 Comment Now! Stephen Edelstein November 8, 2013 Comment Now!

Anyway, when you see a green cap on the air valve of a tire, that means those tires take nitrogen versus regular, 'ol air, according to the Family Handyman website.