road test vs test drive

road test and test drive both are nouns.

Word	Noun	Adjective	Verb	Adverb
road test	Yes	No	No	No
test drive	Yes	No	Yes	No

As nouns, test drive is a hyponym of road test; that is, test drive is a word with a more specific, narrower meaning than road test:

road test: a test to insure that a vehicle is roadworthy
test drive: test of the roadworthiness of a vehicle one is considering buying

road test (noun)	test drive (noun)
a test to insure that a vehicle is roadworthy	test of the roadworthiness of a vehicle one is considering buying

road test (verb)	test drive (verb)
	test a vehicle by driving it