fifth wheel vs car wheel

fifth wheel and car wheel both are nouns.

Word	Noun	Adjective	Verb	Adverb
fifth wheel	Yes	No	No	No
car wheel	Yes	No	No	No

As nouns, car wheel is a hypernym of fifth wheel; that is, car wheel is a word with a broader meaning than fifth wheel:

fifth wheel: an extra car wheel and tire for a four-wheel vehicle
car wheel: a wheel that has a tire and rim and hubcap; used to propel the car

fifth wheel (noun)	car wheel (noun)
a steering bearing that enables the front axle of a horse-drawn wagon to rotate	a wheel that has a tire and rim and hubcap; used to propel the car
an extra car wheel and tire for a four-wheel vehicle
someone or something that is unwanted and unneeded