Is this metonymy: a "set of wheels" to represent a car?
No, it is a synecdoche.
Since wheels are part of a car, the term "set of wheels" is a synecdoche - using a term for part of a thing to stand for the whole thing.
In metonymy, a word or expression strongly linked to something is used in its place. The word "ride" can be used as a metonym for car. It is not part of the car, but can stand for it.