racer vs stock car

racer and stock car both are nouns.

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

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

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

Other hyponyms of racer include finisher.

racer (noun)	stock car (noun)
an animal that races	a racing car with the basic chassis of a commercially available car
slender fast-moving North American snakes	a car kept in dealers' stock for regular sales
a fast car that competes in races
someone who drives racing cars at high speeds