Why do cars with automatic transmissions have tachometers?

The following service procedures made mine a great deal more
responsive, overcoming the characteristics you describe to the point
that the transmission will hold the gears for better acceleration when
desired to the point of maximum safe engine speed, downshift quickly
when appropriate, and upshift early under light acceleration to
maximize fuel economy.
1) put a piece of 2x4 and a concrete block to hold the gas pedal to the
floor, then check the throttle butterfly valve for full opening at WOT.
I found a cable adjustment that needed to move apx. 1/4". The
transmission shift cable was also linked to the same assembly, so once
the cable was adjusted, the trans.cable was fine.
2) factory service manual lists a number of procedures for cleaning the
throttle body and adjusting the throttle position sensor (TPS) and I
performed all of them with the aid of a digital multimeter including an
audile continuity test. The TPS position was off by a few thousandths.
Once the IDL stop was set correctly, the reading at WOT was fine.
3) there are some tiny orofices in the throttle body that direct vacuum
to the EGR vacuum modulator. After following directions and cleaning
with carburetor cleaner, then blowing out the passages with compressed
air, one of the smaller passages emitted a burst of dark colored fluid
- presumably gum had been occluding that port to some degree if not
completely blocking it.
4) adjust throttle stop at vacuum dashpot with vacuum line removed
according to directions.
As stated, transmission now shifts exactly as one would wish, although
it has taken practice to learn the throttle mapping, so as to be able
to feather the downshift points to activate or not as desired - roughly
between 50 - 60 per cent throttle opening.

your model may differ, but I found the original factory marketing
brochures for that year contained graphs for torque and horsepower -
brochures found online.

I noticed on a vehicle years ago with a automatic overdrive transmission
which will dramatically lower engine rpm to about 2000 to 2200 rpm at 60
to 65 mph at a true longer distance cruising speed outside the outer belt
in rural areas.

I read these on a C body Coupe DeVille Cadillac with both (BCM) body
control module and (ECM) engine control module engineered with built in
on-board diagnostics with a FWD THM440 xmission and HT4100 V8
transverse engine. The year is not important.

Before the days of Interstates, the best fuel economy was at about 35 mph
due to urban and rural driving habits at $ .12 to $ .17 per gallon.

Very few vehicles had overdrive transmissions, a Uncle once stated a 82
hp.
1939 Plymouth Coupe during WWII he owned got 20 to 21 mpg driving from
Columbus OH to Chanute Field in "Illinois" at 50 to 55 mph. This Plymouth
Coupe had a flathead 6 engine with a 1 barrel carb, leaded fuel, 3 speed
manual with a 4.11 rear end ratio, and 6.00 by 16 bias ply tires!!!!!

Imagine the true meaning of the "KISS" (keep it simple stupid" principle
with
NO fuel injection controlled by onboard computer, NO catalytic converter,
NO AOD transmission problems, and a $ 845 car FOB Destination delivered.

snip

"Rick Brandt" > wrote:
>Dave C. wrote:
>
>> In several cars I've done the math on, these two formulas produce
>> results pretty close to each other. For my own car, it works out to
>> (3200 to 3600RPM) or 3480RPM, depending on which formula I use. Over
>> ten years of driving this car, I have noticed that maximum fuel
>> economy is achieved at 3500 RPM. [snip]
>
>WTF! In my car 3500 RPM puts me at over 80 MPH. What exactly are you driving?
>
>Based on what you're saying here I would seldom get my car out of second gear.

This topic was recently discussed at length in r.a.d. In a
nutshell, Dave C doesn't understand the difference between
engine fuel efficiency (e.g., power out / fuel flow rate)
and vehicle fuel economy (e.g., miles / gallon used).

He is under the delusion that the engine's most efficient
operating rpm will coincide with the vehicle's best fuel
economy speed. I'm still waiting for him to estimate my
car's fuel economy at 125 mph (at his recommended rpm).

If you follow his extremely poor advice, the result will be
excessive cruising fuel consumption.

If you drive a modern car (roughly 1990-up, but true for
most older cars too) in good running condition, save fuel by
cruising in top gear when possible. Simple.
--
Chuck Tomlinson

Dave C. wrote:
> > >
> > > Aim for RPM at (60% of RPM at which engine produces maximum horsepower)
> > >
> >
> > OK, I have to ask what the goal is by maintaining an artificially high
> > rpm all the time. If you are running along at 3200 or 3600 rpm when
> > you could be in a higher gear you are wasting gas and inducing more
> > wear on the engine.
> >
>
>
> Tell that to your local legislators who thought it necessary to mandate that
> you waste fuel in order to minimize safety and maximize revenue to the local
> governments and maximize carnage on our highways. In other words, you are
> looking at this wrong. I'd gladly upshift and drive safely at the most
> fuel-efficient speed of my engine in overdrive gear, if it was legal for me
> to do so. [Often, I do so anyway. ] But at 78MPH, I am risking my
> license by driving in a very safe, sane, logical and fuel-efficient manner
> in over-drive.

What the hell does 78mph have to do with anything. Put the car in
drive and most likely at 60mph the engine will be turning a very fuel
efficient 2,000 rpm or so. Push the car to 78 or whatever magic number
gets you off and the car will be much much less fuel efficnent. In
simple language even you should be able to understand the car will burn
more fuel to traverse the same distance at 78mph as it will at 60mph.
That is a well established fact.


>
> Yes, I could do 55MPH in 4th gear everywhere. I would get decent fuel
> economy that way. But I'd have to put up with the extra engine noise
> generated at 3500RPM, while it's taking forever and a day to get anywhere.
> Plus, it's harder to hold a constant speed in 4th gear.

Oh gawd, it is YOU again. The brainchild who thinks he understands
engine efficiency.

Because I'm lazy, I
> generally shift up to overdrive even at relatively low speeds like 55MPH.
> But then, I pay for my laziness at the gas pump. The only time I get really
> good gas mileage is when I'm in a hurry and not as worried about getting
> hassled by the cops. IF I can maintain an average speed close to 78MPH for
> long enough, my MPGs go way up. But again, that's not exactly legal in most
> areas of the U.S. -Dave

Charge wrote:
> I noticed on a vehicle years ago with a automatic overdrive transmission
> which will dramatically lower engine rpm to about 2000 to 2200 rpm at 60
> to 65 mph at a true longer distance cruising speed outside the outer belt
> in rural areas.
>
> I read these on a C body Coupe DeVille Cadillac with both (BCM) body
> control module and (ECM) engine control module engineered with built in
> on-board diagnostics with a FWD THM440 xmission and HT4100 V8
> transverse engine. The year is not important.
>
> Before the days of Interstates, the best fuel economy was at about 35 mph
> due to urban and rural driving habits at $ .12 to $ .17 per gallon.
>
> Very few vehicles had overdrive transmissions, a Uncle once stated a 82
> hp.
> 1939 Plymouth Coupe during WWII he owned got 20 to 21 mpg driving from
> Columbus OH to Chanute Field in "Illinois" at 50 to 55 mph. This Plymouth
> Coupe had a flathead 6 engine with a 1 barrel carb, leaded fuel, 3 speed
> manual with a 4.11 rear end ratio, and 6.00 by 16 bias ply tires!!!!!
>
> Imagine the true meaning of the "KISS" (keep it simple stupid" principle
> with
> NO fuel injection controlled by onboard computer, NO catalytic converter,
> NO AOD transmission problems, and a $ 845 car FOB Destination delivered.
>
> snip

Most cars today use torque converter lockups to bring rpms down to
around 2,000 at 60mph and be reasoably fuel efficient. Most manual
transmissions today have a built overdriven 5th gear if that is of any
help. Whether the car uses an overdrive or not is not as important to
fuel economy as how factrs like the overall drive ratio, number of
gears available, the circumference of the wheels, the engine power and
size of the vehicle are tied together.

In article >,
Dave C. > wrote:
(snip)
>Aim for 40-45% of engine redline
>
>Aim for RPM at (60% of RPM at which engine produces maximum horsepower)
(snip)

Interesting. My car's redline is 7000rpm, and they quote the
horsepower at 6000rpm. So, the redline suggests a target of
2800-3150rpm, and the horsepower a target of 3600rpm. Sounds
like I should be cruising in a lower gear than I have been.

(snip)
>Unfortunately, it might not be that easy if you own a slushbox. But I
>gathered from what you wrote that you have a real tranny, so you should be
>all set. -Dave

Mmmm, thanks. I may give that a try, and I'll also look into
what the equivalent stats are for my wife's car!

-- Mark

In article >,
Chuck Tomlinson > wrote:
(snip)
>This topic was recently discussed at length in r.a.d. In a

Ah, thanks - I'll check the archives on Google.

>nutshell, Dave C doesn't understand the difference between
>engine fuel efficiency (e.g., power out / fuel flow rate)
>and vehicle fuel economy (e.g., miles / gallon used).

Mmmm - I remember my high school math teacher once made the same
mistake in a problem the class worked on!

(snip)
>If you follow his extremely poor advice, the result will be
>excessive cruising fuel consumption.

Also, John mentions engine wear, another worrying issue.

>If you drive a modern car (roughly 1990-up, but true for
>most older cars too) in good running condition, save fuel by
>cruising in top gear when possible. Simple.

That is simple. (-: It also matches what I'm used to others doing.
Thanks.

-- Mark

In article >,
Mark Carroll > wrote:
>One thing I wish I had is a torque-speed curve and fuel efficiency map
>and whatever for my engine. I don't know where its most efficient
>points are, partly because I can't find that information out. Are
>there any good sources of this type of advice?

In at least some cars and some scan tools (that plug into the OBD port),
you can get fuel consumption readings at various speeds and gears. You
can record them and then calculate which speeds and gears give the best
fuel economy.
--
------------------------------------------------------------------------
Timothy J. Lee
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