system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What’s the use of tachometer?

While driving my car, I use speedometer to see the speed but never check and understand the tachometer. I keep thinking it occupies lot of space unnecessarily.
gpt
For most cars with automatic transmissions, the tachometer is largely useless except as a way to indicate a problem with the transmission or other part of the drivetrain (although on modern cars, the computers cover that regardless).  On manual (aka standard) shift cars, a tachometer can provide a good visual cue to know when to shift up or down, although most experienced drivers rely on feel more than the visual of a tachometer.  In high end sports or performance cars or heavy duty trucks, a tachometer provides precise information that tells a driver how hard they are pushing the engine.  But in most cars for most conditions, they are pretty much useless.