Speedometer

wiktionary

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speed +‎  -o- +‎  -meter

etymonline

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speedometer (n.)

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.