roadster vs motorcar

roadster and motorcar both are nouns.

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

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

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

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

roadster (noun)	motorcar (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