car horn vs automobile horn

car horn and automobile horn both are nouns.

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

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

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

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

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