When one person (said person can be single/taken/etc) hangs out with multiple couples at once, and can no longer be referred to as a “Third Wheel.” A “Third Wheel” usually applies to one person and ONE couple.
Person 1: “Yea man, I was at the movies with Shaun, Nick, and their girlfriends, it was pretty fun!”
Person 2: “You had fun? You weren’t even a “Third Wheel,” you hung out with two couples, that makes you a “Spare Tire.”

Person 3: “Hey!…Don’t worry man, you’ll find someone soon enough!”
by LookHoneyAnAbomination June 7, 2022
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the friend that you bring with you to a hangout as a "third wheel" when you're unsure if the hangout is platonic or romantic in nature.
"This guy asked me to hangout but I don't know if he meant it as a date. I need you to be my spare tire so it's clear we're just friends." "Can you be my spare tire?"
by tunasub_backwards February 7, 2022
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