Personally I think we should merge the two tags because they appear to be used for the same kind of questions, but the final decision is up to the community. Everyone with 1250 or more reputation can suggest tag synonyms and/or approve suggestions. If we want to merge the two then we need to decide what the main tag will be; cars or car. Either is fine by me. We don't seem to be consistent with singular or plural when it comes to tags (for example batteries, building-materials, electric-vehicles but also vehicle, plastic, green-roof), although I think plural is a bit more common than singular.