kilometers per hour vs kilometres per hour

kilometers per hour and kilometres per hour both are nouns.

Word	Noun	Adjective	Verb	Adverb
kilometers per hour	Yes	No	No	No
kilometres per hour	Yes	No	No	No

As nouns, kilometers per hour and kilometres per hour are synonyms defined as:

kilometers per hour and kilometres per hour: the ratio of the distance traveled (in kilometers) to the time spent traveling (in hours)

Other synonyms of kilometers per hour include km/h, kph.

kilometers per hour (noun)	kilometres per hour (noun)
the ratio of the distance traveled (in kilometers) to the time spent traveling (in hours)	the ratio of the distance traveled (in kilometers) to the time spent traveling (in hours)