A tachometer measures how fast the engine is spinning. It displays the engine speed in RPM — revolutions per minute.
What it’s used for
The tachometer helps you:
Know when to shift gears in a manual car 
Avoid over-revving the engine 
Drive more efficiently 
Monitor engine behavior 
Why it matters
In a manual transmission car, the tachometer is very useful because it helps you shift at the right time:
Lower RPM = better fuel economy 
Higher RPM = more power and acceleration 
In automatic cars, the computer shifts for you, but the tachometer still shows how hard the engine is working.