speed-measuring instrument, 1810, coined by inventor, Bryan Donkin, from tacho- "speed" + -meter. Related: Tachometry.
1904, from speed + -meter. A Germanic-Greek hybrid and thus much execrated.
[T]he ancient Greeks & Romans knew what speed was, & yet no-one supposes they called it speed, whence it follows that speedo- & speedometer are barbarisms. [Fowler]
The correct classical formation is tachometer.