You've rightly identified that ancestor relationships in GAE should not be based on the logical structure of your data.
They need to be based on the transactional behavior of your application. Ancestors make your life difficult. For example, once you use a compound key, you won't be able to fetch that entity by key unless you happen to know all the elements of the key. If you knew the Car id, you wouldn't be able to fetch it without also knowing the other component.
Consider what queries you would need to have strong consistency for. If you do happen to need strong consistency when querying all the cars in a given branch, then you should consider using that as an ancestor.
Consider what operations need to be done in a transaction, that's another good reason for using an entity group.
Keep in mind also, you might not need any entity group at all (probably the answer for your situation).
Or, on the flip side, you might need an entity group that might not exactly fit any logical conceptual model, but the ancestor might be an entity that exists purely to exists because you need an ancestor for a certain transaction.