police car vs auto

police car and auto both are nouns.

Word	Noun	Adjective	Verb	Adverb
police car	Yes	No	No	No
auto	Yes	No	No	No

As nouns, auto is a hypernym of police car; that is, auto is a word with a broader meaning than police car:

police car: a car in which policemen cruise the streets; equipped with radiotelephonic communications to headquarters
auto: a motor vehicle with four wheels; usually propelled by an internal combustion engine

Other hypernyms of police car include automobile, car, machine, motorcar.

police car (noun)	auto (noun)
a car in which policemen cruise the streets; equipped with radiotelephonic communications to headquarters	a motor vehicle with four wheels; usually propelled by an internal combustion engine