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According to the dictionary, both of these terms refer to the same object: car/automobile.
Now, I understand that 車 is also used in compounds to provide the "vehicle" part as in 電車, but I'm just referring to the meaning "car" in this question.
Is there really a difference between these two terms? Register? Formal/informal? None?
Nowadays, 自動車 is used in much the same context as "automobile", just as 車 is used in the same way as "car". Indeed, the kanji that make up 自動車 literally translate to "automobile". Generally speaking, if you can't decide which is more appropriate to use in a given situation, just imagine if in English you would say "automobile" or "car".
That said, 自動車 has more of a technical connotation to it. You'll primarily see it in technical documents, official forms (such as insurance forms), and the names of companies (e.g. 株式会社西田自動車). "Car" is more common in casual speech.
車 has the meaning of something that moves via wheels. For example before the cars ever existed, when people used the word "車" they almost always were referring to 牛車.
自動車 as its name implies means something that uses wheels (like 車) and those wheels are driven using an engine or motor.
Nowadays, 車 always refers to 自動車 and is more commonly used in speech。
