Short for tachometer; a gauge in a vehicle that shows how fast the engine is spinning (measured in RPM).
From Greek 'tachos' (speed) + '-meter' (measuring device). The 'tach' is a shortened form used as slang among mechanics and drivers.
The tach tells you something your engine's speed, but experienced drivers learn to feel engine speed from engine sound and vibration — race car drivers often drive without looking at the tach because they've tuned their senses to the engine's rhythm.
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