Why do cars with automatic transmissions have tachometers?

A tachometer on an automatic transmission vehicle can be very useful, especially for one that is electronically controlled.

If I want to get the transmission to shift smoothly at something like 3000 rpm in each gear, I can gradually accelerate and then let off the throttle just a bit as the tachmeter nearly reaches 3000 rpm, at which time, the transmission will shift at the desired RPM's.

Same goes for if I want the transmission to shift at 2500 RPM's or less (although that's really slow acceleration generally only used in residential areas) or at 4000 RPM's or more (climbing a bit of a grade). The accurate shifts at a given RPM can continue to occur until the transmission reaches its final gear and torque converter lockup subseqently occurs, thanks to the visual feedback of the tachometer.

After a while, I've learned the acceleration rates for shifting at a given RPM, so I can maintain the throttle in a constant position such that the transmission shifts smoothly at the desired RPM.

Without a tachometer, I could judge by engine sound, but not know the exact RPM's. And, it's also helpful to know when the engine is approaching redline under more spritied acceleration (e.g., flooring it on a very short interstate onramp that merges rather than joins with an extra lane, giving very little distance to get up to the 65mph freeway speed) rather than just waiting for the transmission to automatically shift to the next gear when the engine hits redline.

Since my car's specifications indicate a maximum of 126 horsepower at 5500 RPM, there's really no need to exceed 5500 RPM, but I wouldn't know if I reached or exceeded that max engine torque RPM under higher acceleration without a tachometer.

Others may vary in their usage or requirements for a tachometer.

As Tom said, to make you think you got a sporty car. Useless as an udder on a turtle for most drivers.

Same here. I use mine all the time for forcing up-shifts and as a general check on engine performance. Example: Hmmm...last month I was climbing this hill in 6th (od), what happened that I am now in 5th?. I can't feel or hear the diffeence in the 6/5 or 5/6 shift so see of the pants is no indicator.

Harry K

TLittle wrote in news: snipped-for-privacy@mail.net:

ive found factory tach's handy in diag. some questionable tranny shift probs.without having to hook up the scanner...shift points, shift slip, etc....i think your first RE; had it pegged, just another 'do-dad' like auto vol. control and steering wheel radio controls....as much as they say there cars are values they sure spend alot of time and money on crap thats not needed.........................kjun

I found it useful to diagnose a TCC problem in my X-car, I could easily see what RPMs the clutch locked up and unlocked, and when I noticed that it wasn't locking and unlocking at the proper RPMs for the conditions I knew something was up. Sure enough, a few thousand miles later it failed, but thanks to the tach I had plenty of warning and was prepared to immediately repair the problem.

JazzMan

And from what I've read in car buyer's guides, automatic transmissions are a lot less reliable than manual. So in a way it makes sense to put them on cars with automatic transmissions.

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For me, it's a similar situation to a car with a shift light ... I can tell by the engine sound and pulling power when it's time to shift. I guess some people can't, though. I'm waiting to see a car with an automatic trans and a shift light ... :)

To use current-day terminology: it's "bling".

Because it's nice to know when your getting ready to "redline" when you have your foot all the way to the floor.

Unless you want to know when you're getting ready to redline when the foot is to the floor.

It tends to help diagnose problems with one's transmission.

-LMB

Well, if you want to hold a low gear for acceleration up a hill or something, it's nice to know how hard you're thrashing the engine.

Or setting your idle speed....

Tell you what, why do cars have oil pressure warning lights? I mean, you know if your car loses all oil pressure, right? The engine grinds to a halt and seizes... Why does your car have a temperature gauge? You'd know if it was overheating by the clouds of steam...

It's getting hard to find cars with oil pressure gauges today, and even BMW is now promoting fake temperature gauges. Soon, even the idiot lights will all be replaced by one red light that says REPLACE ENGINE.

--scott

We read what you had to say the first two times, idiot.

Because it's nice to know when your getting ready to "redline" when you have your foot all the way to the floor.

just had to say it too. :)

Becasue the car makers think people want them. I like having one even if I don't need one.

Ed

A better question would be, "Why do modern cars have useless instruments like a Tach, but lack useFUL instruments like oil pressure gauges calibrated in PSI, voltmeters calibrated in volts, or better yet ammeters." Oil temp and transmission temp gauges would be kinda nice too, especially when you're towing. The WORST of all are cars with a tach and NO other gauges, just idiot lights.

FWIW- only one of my automatic cars (1993 Vison TSi) has a tach, and no I never "use" it. It is kinda nice to see when the torque convertor locks up to help save a little gasoline, but that's about it.