hot rod vs auto

hot rod and auto both are nouns.

Word	Noun	Adjective	Verb	Adverb
hot rod	Yes	No	No	No
auto	Yes	No	No	No

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

hot rod: a car modified to increase its speed and acceleration
auto: a motor vehicle with four wheels; usually propelled by an internal combustion engine

Other hypernyms of hot rod include automobile, car, machine, motorcar.

hot rod (noun)	auto (noun)
a car modified to increase its speed and acceleration	a motor vehicle with four wheels; usually propelled by an internal combustion engine