secondhand car vs auto

secondhand car and auto both are nouns.

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

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

secondhand car: a car that has been previously owned; not a new car
auto: a motor vehicle with four wheels; usually propelled by an internal combustion engine

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

secondhand car (noun)	auto (noun)
a car that has been previously owned; not a new car	a motor vehicle with four wheels; usually propelled by an internal combustion engine