overhaul vs improvement

overhaul and improvement both are nouns.

Word	Noun	Adjective	Verb	Adverb
overhaul	Yes	No	Yes	No
improvement	Yes	No	No	No

As nouns, improvement is a hypernym of overhaul; that is, improvement is a word with a broader meaning than overhaul:

overhaul (noun)	improvement (noun)
periodic maintenance on a car or machine	the act of improving something
the act of improving by renewing and restoring	a change for the better; progress in development
	a condition superior to an earlier condition

overhaul (verb)	improvement (verb)
make repairs, renovations, revisions or adjustments to
travel past