Nothing is better than getting a free tank of gas. It is one of the best experiences one can have aside from a threesome, which in turn could be described by the phrase. It may also be used to describe the greatness of a person, place, or thing.
That party was a free tank of gas!

That girl's body is a free tank of gas but her face was diesal.
by Turd Ferguson. July 2, 2008
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