What is the difference between a synecdoche and a metonymy?

Synecdoche refers to a thing and its parts. The best example is the use of the word "wheel" instead of "car". As we know, a wheel is a car part.

On the other hand, metonymy refers to a thing that is very much related to another thing. The easiest example we can use here is the word "crown" when used in some literature pieces means power.