I’ve had a tachometer in a couple of my vehicles, and never knew what it was for, or what I should do with it. I just ignore it.
There must be a reason for it?
(And, for some reason Discourse is giving me a message: “Your topic is similar to … I’m now a card-carrying member of the A.C.LU.!”) Say what?
Weirdly, I see it recommended as important for vehicles with manual transmission so you know when to shift. I have never paid attention to one when driving with a manual; I just listen to the engine sound.
I thought a tach was for drivers who see it as a must-have cool gauge in a sports car.
*there’s probably a negative correlation between having a tachometer and being an ACLU member.
That bit about shifting I’ve heard about. But I only drive automatics. Why would they install them in automatics?
If you find out, let me know. I never had a tach in any manual car I owned, but as soon as I started buying automatics a tach was standard. This goes back decades, although my last couple of cars finally stopped the nonsense. Always baffled me.
(And Discourse’s “similar to” nagging is as baffling as the tach situation and far more irritating.)
Sometimes it’s handy for diagnosis, for example, “yeah, the buzzy noise starts at 3000 rpm, regardless of speed.”
Unless you are actually racing, pretty much nothing. Looks cool. Balances the speedometer and forms a nice double 0 shape.
I have had a tach in every car I owned. I tend to watch it quite a bit.
In manuals it helps precision when changing gear. In an auto it is helpful as it gives feedback as to how the auto gearbox is behaving. This is especially true at low revs, say when taking off on a slippery slope or pulling a trailer. Just relying on engine note is much less precise and in a modern car hard to hear. (Especially with the music turned up.)
Because to save money they don’t design cars differently between automatics and manuals. Rather than have to put in a new dash and gage clusters when they change the transmission they just keep it the same.
I use my tach in my automatic cars to improve my fuel efficiency and try to maintain my rpm in the sweet spot for fuel milage. I did the same thing with my manual cars.
But what if you just want to get in, turn it on, drive somewhere, turn it off, and get out? It’s just a gadget to ignore.
But aren’t the great majority of cars now autos? So the minority vehicle drives the design?
Yup. Cheap is what drives the design so if a special dash for 10,000 cars costs more than putting a tach in a million cars then the million are getting a tach.
As stated above, the tachometer is ideal if you’re manually shifting, especially if you’re racing. You’ll wring maximum power out of your engine when you’re at the edge of the redline (that solid red stripe on the far end of the tachometer) If you’re driving for economy, you’ll want to shift well before then.
Some automatics will let you shift manually anyway, usually by selecting a little notch off to the side (if you have a classic PRNDL-type affair) or if you have shift paddles on the wheel.
So to reiterate, the tachometer’s primary purpose is to let you, the driver, decide the optimal time to shift. If you’re not making those decisions, you don’t need the thing. It looks sporty.
If your journey is a trivial one, relatively flat ground, good traction, not much load in the car, sure fine. But add a trailer, or load the car up with passengers and gear, find yourself starting on a steep incline in the rain. There are plenty of times when understanding and using a tacho will improve your driving. Going to the shops is probably not one of them. But manufacturers can reasonably assume you will occasionally drive the car for more demanding chores. There are times when what you want and what you get don’t align.
doesn’t happen in Saskatchewan. 
I’ll add. Automatics, even if you leave them alone are not magical, and their choice of change point is driven by a mixture of factors. Throttle position and revs come into play. Watching the tacho in an auto can allow you to manage the gear change points in difficult conditions using the throttle. It also allows you to understand how the torque converter is behaving (if your auto has one.) Any time you are in limited traction conditions the tacho is your friend. In poor traction the tacho is what tells you what is going on no matter what sort of gearbox you have.
When you have a large load the tacho will tell you how things are going as well.
What’s interesting is that in my car, the “instrument cluster” in the dash is entirely digital. It’s just a screen, and you can configure it to display different things in different ways–but AFAIK, there’s no way to make it NOT display a tachometer. And the car only comes in automatic.
An automatic still has one or two manual low gears. The tach is handy when you are on a particularly steep grade or towing something.
In racing, a tachometer is far more important than a speedo. Many race cars don’t even bother with a speedo. Just a big tach. The point is not only to know when to shift for maximum performance. But also so you don’t blow up your motor. If you’re not racing, or otherwise completely incurious about such things, don’t drive like an idiot and you can ignore it.
Having never driven (and almost never even been in) a manual car, what about a tachometer tells you when to shift?
On some level it’s like fog lights, or dual exhaust tips, or body colored side mirrors. The tachometer used to be a factory installed option on a lot of cars, and not having it meant you were cheap or poor or generally unworthy. Eventually it became standard equipment because not having one was a signal to the buyer that the car was cheap.
On another level, it’s a thing that does what it does. Just like fog lights. 99.99% of car fog lights will never get used. Maybe they’ll get turned on, maybe they’re turned on all the time, but if they didn’t exist we’d all get along just fine. The tachometer tells the driver what speed the engine is running at at any given time. We can come up with scenarios where that might be useful (e.g. diagnosing a problem), but if every tachometer disappeared overnight we’d all get along just fine.