rental vs rent-a-car

rental and rent-a-car both are nouns.

Word	Noun	Adjective	Verb	Adverb
rental	Yes	Yes	No	No
rent-a-car	Yes	No	No	No

As nouns, rent-a-car is a hyponym of rental; that is, rent-a-car is a word with a more specific, narrower meaning than rental:

Other hyponyms of rental include car rental, hire car, self-drive, u-drive, you-drive, sublease, sublet.

rental (noun)	rent-a-car (noun)
the act of paying for the use of something (as an apartment or house or car)	a rented car
property that is leased or rented out or let

rental (adjective)	rent-a-car (adjective)
of or relating to rent
available to rent or lease