Editing Tachometer (section)

{{Short description|Instrument measuring the rotation speed of a shaft or disk}}
{{Distinguish|text = [[Tachograph]], used for recording vehicle data, [[Tacheometer]], used for surveying, or [[Tachymeter (watch)]], used on watches}}
{{Redirect|Revolution-counter|opposition to a fundamental change in power|Counter-revolution}}
{{Redirect|Tach|other uses|Tach (disambiguation)}}
{{More citations needed|date=October 2010}}
[[Image:Speedometer.jpg|thumb|300px|A tachometer that can indicate up to 7000 [[revolutions per minute|RPM]] (left)]]

A '''tachometer''' ('''revolution-counter''', '''tach''', '''rev-counter''', '''RPM gauge''') is an instrument measuring the [[rotation speed]] of a [[axle|shaft]] or disk, as in a motor or other machine.<ref>{{cite book |last=Erjavec |first=Jack |title=Automotive Technology |year=2005 |publisher=Thomson/Delmar Learning |isbn=1-4018-4831-1}}</ref> The device usually displays the [[revolutions per minute]] (RPM) on a calibrated analogue dial, but digital displays are increasingly common.

The word comes {{ety|grc|''τάχος'' (táchos)|speed||''μέτρον'' (métron)|measure}}. Essentially the words tachometer and [[speedometer]] have identical meaning: a device that measures speed. It is by arbitrary convention that in the automotive world one is used for engine revolutions and the other for vehicle speed. In formal engineering nomenclature, more precise terms{{Which?|date=January 2025}} are used to distinguish the two.