fire truck vs fire engine

fire truck and fire engine both are nouns.

Word	Noun	Adjective	Verb	Adverb
fire truck	Yes	No	No	No
fire engine	Yes	No	No	No

As nouns, fire truck and fire engine are synonyms defined as:

fire truck and fire engine: any of various large trucks that carry firemen and equipment to the site of a fire

fire truck (noun)	fire engine (noun)
any of various large trucks that carry firemen and equipment to the site of a fire	any of various large trucks that carry firemen and equipment to the site of a fire