stock car vs auto

stock car and auto both are nouns.

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

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

stock car: a car kept in dealers' stock for regular sales
auto: a motor vehicle with four wheels; usually propelled by an internal combustion engine

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

stock car (noun)	auto (noun)
a racing car with the basic chassis of a commercially available car	a motor vehicle with four wheels; usually propelled by an internal combustion engine
a car kept in dealers' stock for regular sales