roadster vs automobile

roadster and automobile both are nouns.

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

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

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

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

roadster (noun)	automobile (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

roadster (verb)	automobile (verb)
	travel in an automobile