petrol tank vs storage tank

petrol tank and storage tank both are nouns.

Word	Noun	Adjective	Verb	Adverb
petrol tank	Yes	No	No	No
storage tank	Yes	No	No	No

As nouns, storage tank is a hypernym of petrol tank; that is, storage tank is a word with a broader meaning than petrol tank:

petrol tank: a tank for holding gasoline to supply a vehicle
storage tank: a large (usually metallic) vessel for holding gases or liquids

Other hypernyms of petrol tank include tank.

petrol tank (noun)	storage tank (noun)
a tank for holding gasoline to supply a vehicle	a large (usually metallic) vessel for holding gases or liquids