[kqr]

Both kph and mph sort of follow a pattern of the first letter of each word in the unit being spelled out, but kmph reads ambiguously as k(mph) or (km)ph.

Of course, anyone caring about correctness would use the SI unit symbol, which is m/s.

[kelipso]

km is very standard abbreviation for kilometers. Like anyone outside the US would recognize that and consequently kmph. Of course, km/h is very recognizable and k/h looks like nonsense. So I'd say kph is the objectively terrible option.