racing car vs auto

racing car and auto both are nouns.

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

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

racing car: a fast car that competes in races
auto: a motor vehicle with four wheels; usually propelled by an internal combustion engine

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

racing car (noun)	auto (noun)
a fast car that competes in races	a motor vehicle with four wheels; usually propelled by an internal combustion engine