A device for measuring the revolutions per minute (RPMs) of a revolving shaft, as with the driveshaft of an automobile.
It’s the thing on the left here, measuring the RPMs of the engine, which is to say how hard the engine is working. Which IIRC is how you can tell when to shift when driving a manual transmission.
Wikipedia: “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 and the other for vehicle speed.”
Personally I’d have more use for a taco-meter. 🌮
He looked back like a nocturnal rabbit trying to outstare the headlights of a sixteen-wheeled artic whose driver is a twelve-hour caffeine freak outrunning the tachometers of hell.