truck vs tank truck

truck and tank truck both are nouns.

Word	Noun	Adjective	Verb	Adverb
truck	Yes	No	Yes	No
tank truck	Yes	No	No	No

As nouns, tank truck is a hyponym of truck; that is, tank truck is a word with a more specific, narrower meaning than truck:

truck: an automotive vehicle suitable for hauling
tank truck: a motor vehicle designed to carry liquefied loads, dry bulk cargo or gases on roads.

truck (noun)	tank truck (noun)
an automotive vehicle suitable for hauling	a motor vehicle designed to carry liquefied loads, dry bulk cargo or gases on roads.
a handcart that has a frame with two low wheels and a ledge at the bottom and handles at the top; used to move crates or other heavy objects

truck (verb)	tank truck (verb)
convey (goods etc.) by truck