automobile horn vs car horn

automobile horn and car horn both are nouns.

Word	Noun	Adjective	Verb	Adverb
automobile horn	Yes	No	No	No
car horn	Yes	No	No	No

As nouns, automobile horn and car horn are synonyms defined as:

automobile horn and car horn: a device on an automobile for making a warning noise

Other synonyms of automobile horn include hooter, horn, motor horn.

automobile horn (noun)	car horn (noun)
a device on an automobile for making a warning noise	a device on an automobile for making a warning noise