Modern car dashboards have numerous indicators and controls. Some are necessary: without a fuel gauge, for instance, we’d be in frequent trouble. Others are optional but quite useful: the rear collision warning beeper is a good recent arrival. But one indicator is totally unnecessary, yet present in a great many car models: the Tachometer, or engine RPM indicator.
This meter is very impressive, to be sure, and on a race car would be quite useful to help the driver wring the ultimate performance without destroying the engine. But this meter, at left in the photo, is not from a Formula 1 car; it’s from my faithful but mundane Mazda 3, with which I navigate the congested roads daily to get to work. You think I constantly glance at the tachometer to set the gears so my engine’s not redlining, when I’m stuck in traffic jams half the time? Even if I were so inclined, this car has an automatic transmission!
Fact is, nobody uses this indicator on a family car; yet the manufacturers put them in - adding just a little bit extra to the waste of energy and resources needed to produce the car. I suspect this is another of the cases where fashion, vanity and marketing get together to override commonsense design - we get this meter because it looks flashy, fashionable, macho… hey, I have an RPM gauge like the race track pros!
Millions made - none needed 
Tachometers are close to useless on automatic transmission cars, but they do serve on purpose - to let you know, visually, what gear you’re in. That is, on my automatic transmission car, I can glance at the tach, and know which gear I’m in, depending on my speed. There’s no other way to know.
And on a manual transmission, a tach is much more useful, for deciding if you should shift gears (granted, it’s not hugely important in normal driving, and you could get along without one). I’m the kind of person who likes to know what my car is doing in every way - the kind of person who wants to know what my CPU and RAM utilization are at any given moment on my computer, as clues to what it’s doing, and if it’s working normally.
Besides, it’s probably harder and more costly for car makers to make completely different dashboard instrument panels for different versions of their cars - one with a tach for manual transmissions, and one without for automatics. It’s easier and cheaper just to include it, for the people who care, and for the people that don’t, well, they get to be irked by how wasteful and needlessly complex they are.
Actually, I do use the tachometer.
In my motorcycle, for instance, the speedometer is significantly less important - I am much more interested in the RPM than in my speed.
Same goes for my wife’s car. But there is a difference from your case - we have (and prefer) manual transmission. I want to shift the gear whenever I want to, not whenever the machine ‘thinks’ it should. And the best way to know when to shift is the tachometer.
I agree that the tachometer isn’t needed for automatic transmission. But in my opinion, so is the driver - If I wanted somebody else to shift the transmission, I’d probably want somebody else to steer the wheel. It’s called a Taxi Driver.
Ahhh… some controversy!
Of course, you’re right, Josh and Asa… in a manual car it makes more sense, and those with a passion for cars (and, even more, motorcycles!) would indeed make use of this dial. Though IIRC, didn’t one use one’s ear to keep track of the right moment to shift? I do miss the manual shift, but with traffic jams being so common, the benefits kinda paled…
Incidentally, on my automatic you still don’t need the tachometer to tell what gear you’re in - Mazda used a belt-and-suspenders approach and threw in a digital readout of the current gear as well. No amount of complexity is too good for our customers!
actually, when i race my corvette on the track, the tach is used and the speedo is never used. most special purpose race cars i’ve ever seen do not have a speedo. around town, i use the tach to control gas mileage.
i also race in something called auto-cross, which uses stock and normal daily driver cars as track cars, and i don’t think tachs are useless there either. it would be hard to make them optional on normal cars, and the manufacturer can’t know what the end use is going to be.
Tacho is most usefull thing on a dashboard in manual shift vehicle, even in everyday trafic. I would rather sacrifice fuel consumption gauge, which is quite useless anyway, than a Tacho. Altough in your average MPV you could put a smaller version, it does not have to be that big and take up big part of dashboard.
Another critical use for a tach is if the car is ever stuck. If the wheels are pulling without resistance (ice/mud) you need to be careful not to over-rev the engine when trying to get out.
The Tach is also useful for diagnosis. I can tell some thing about my engine by the RPM it idles at or I can tell if the AC compressor has just kicked in, or if it’s about to stall. It is useful when shifting as many modern cars have very good sound insulation. I can tell when my tires have broken loose in the snow. It is necessary to prevent over-revving while engine braking. Also, some of use drive spiritedly everyday.
I stumbled across this letter looking for more tach information. I have
an automatic now and I ordered a tach today. I don’t hear as well as some people and I need the tach to tell me what the wheels are doing on ice &snow and to tell me if the transmission is slipping before it gets bad and costly to repair. The people who make cars have a little more on the ball that the author of tachs are wasted space. Kipp