The best replacement for mph is knots.

The best alternatives to mph are rated as following:
★★★★★  knots
★★★★★  mha
★★★★  kph
★★★  mba
★★  mpa
★★  kilometers
★★  msph
★★  md
★★  km
★  mph
★  fps
★  ms
★  kph conversion
★  knot
★  rpm
★  msn
★  msw
★  minutes per mile
★  kmh
★  mpg
  ms in epidemiology
  km h
  kph calculator
  mpp
  m p h
  km hr
  feet per minute
  kilometers per hour
  m.p.h
  cfm
  kmp
  kmph
  et
  msc
  kts
  knots per hour
  kilometer
  knotts
  mhs
  mach
  feet per second

Your alternative for mph is not listed? Suggest!
Speaking about mph, one probably shouldn't forget about md.

Other search terms related to mph and their ratings are:
★★★★★ md
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Google Trends

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mph
knots
mha