A tachometer is a device in a car that shows how fast the engine is working.
A tachometer is a useful thing for people who drive cars or work on cars.
Examples:
The tachometer in the car shows how fast the engine is going.
I looked at the tachometer to see if the engine was working hard.
The tachometer is next to the speedometer in the car.
The tachometer helps me know when to change gears.
The tachometer can tell you if the engine is working very, very fast.
The tachometer is like a speedometer, but for the engine.