racing car vs stock car

racing car and stock car both are nouns.

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

As nouns, stock car is a hyponym of racing car; that is, stock car is a word with a more specific, narrower meaning than racing car:

racing car: a fast car that competes in races
stock car: a racing car with the basic chassis of a commercially available car

Other hyponyms of racing car include finisher.

racing car (noun)	stock car (noun)
a fast car that competes in races	a racing car with the basic chassis of a commercially available car
	a car kept in dealers' stock for regular sales