speedometer vs tachometer

speedometer

noun
  • Such a device incorporating an odometer. 

  • An instrument located within the dashboard of a vehicle that measures and indicates the current speed of the vehicle. 

tachometer

noun
  • A device for measuring or indicating velocity or speed, as of blood, a river, a machine, etc. 

  • A device for measuring the revolutions per minute (RPMs) of a revolving shaft, as with the driveshaft of an automobile. 

How often have the words speedometer and tachometer occurred in a corpus of books? (source: Google Ngram Viewer )