It’s a bit more complex than that. You would need to hook up the a filter that will allow you modify the values that the relationship field has:
https://www.advancedcustomfields.com/resources/acf-prepare_field/
Once you are in there, you create a query that will load only those posts that have the car tag and then return the value. When the post loads, it will only display those posts.
It depends on how your code is setup and names of variables, so it’s going to be a simple copy/paste.
