rent-a-car vs rental

rent-a-car and rental both are nouns.

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

As nouns, rental is a hypernym of rent-a-car; that is, rental is a word with a broader meaning than rent-a-car:

Other hypernyms of rent-a-car include lease, letting.

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

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