Google has updated its product structured data help documents to say that you need both product and car structured in order for Google to display ratings and review snippets in search for car search results.
Google said "currently car is not supported automatically as a subtype of Product." "So for now, you will need to include both Car and Product types if you would like to attach ratings to it and be eligible for the Search feature," Google added. Here is a screenshot of this new note added to the help document:
Google noted this over here saying "added a note describing how to specify Car markup and still have Product review snippet feature eligibility."
Forum discussion at Twitter.
