roadster vs auto

roadster and auto both are nouns.

Word	Noun	Adjective	Verb	Adverb
roadster	Yes	No	No	No
auto	Yes	No	No	No

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

roadster: an open automobile having a front seat and a rumble seat
auto: a motor vehicle with four wheels; usually propelled by an internal combustion engine

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

roadster (noun)	auto (noun)
a small lightweight carriage; drawn by a single horse	a motor vehicle with four wheels; usually propelled by an internal combustion engine
an open automobile having a front seat and a rumble seat