kilometres per hour vs kilometers per hour

kilometres per hour and kilometers per hour both are nouns.

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

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

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

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

kilometres per hour (noun)	kilometers 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)