Use taxonomy but automatically assign the taxonomy. So anytime you create a car, it's automatically assigned to term "transportation." You can do this with the Taxonomy Defaults module.
Taxonomy Defaults allows you to assign default terms from any vocabulary to any node-type. The default terms will be pre-selected on the /node/add form, but can be edited.
If a vocabulary is marked as "Hidden" for a particular content type, then that vocabulary will not be shown on the /node/add form and the default terms will be assigned automatically and cannot be edited.
The module is a bit tricky to get at first, make sure you select "hidden" so that the user can't change the default taxonomy.