First, let me admit that I don’t know much about cars.
I know to step on the gas and turn the steering wheel,
that’s about it. 
I just bought an Altima that has an "Overdrive" with
the automatic. What is this for? All the salesman
can tell me is "use overdrive on the freeway, other-
wise turn it off". But the manual says to "use
overdrive during normal driving conditions, turn it
off for going up/down steep hills" (paraphrased).
So what is overdrive? does it cause the transmission
to switch to a higher gear quickly than if overdrive
is not used? if so, then why is overdrive necessary
when there is already a first and second gear on the
automatic for steep driving?
i tend to believe that i should follow the manual’s
advice, rather than the salesman’s. is this correct?
the manual also mentioned something about "engine
braking". what is it? thanks for the replies. i
know we have some auto wizards on this group.
– lulu
–
—==l…@cs.ucr.edu==— just another overworked graduate student.
David T Lu, Amateur Thinker: l…@cs.ucr.edu, {ucsd, uci}!cs!lulu
"I seem to be having tremendous difficulty with my lifestyle."
– Arthur Dent, Hitchhiker’s
David Lu (l…@ucrengr.ucr.edu) wrote:
: First, let me admit that I don’t know much about cars.
: I know to step on the gas and turn the steering wheel,
: that’s about it.
: I just bought an Altima that has an "Overdrive" with
: the automatic. What is this for? All the salesman
: can tell me is "use overdrive on the freeway, other-
: wise turn it off". But the manual says to "use
: overdrive during normal driving conditions, turn it
: off for going up/down steep hills" (paraphrased).
: So what is overdrive? does it cause the transmission
: to switch to a higher gear quickly than if overdrive
: is not used? if so, then why is overdrive necessary
: when there is already a first and second gear on the
: automatic for steep driving?
: i tend to believe that i should follow the manual’s
: advice, rather than the salesman’s. is this correct?
: the manual also mentioned something about "engine
: braking". what is it? thanks for the replies. i
: know we have some auto wizards on this group.
: – lulu
: —
: —==l…@cs.ucr.edu==— just another overworked graduate student.
: David T Lu, Amateur Thinker: l…@cs.ucr.edu, {ucsd, uci}!cs!lulu
: "I seem to be having tremendous difficulty with my lifestyle."
: – Arthur Dent, Hitchhiker’s
Overdrive is a gear ratio higher than 1:1 (lower numerically). For
example, your first 3 gears might be approx. 2.8:1, 1.8:1, and 1:1;
your overdrive, or 4th gear (in an automatic) might be 0.8:1. When
the trans. shifts into overdrive, the engine is turning fewer rpms,
which means better gas mileage and less noise.
The trans. normally will shift down into 3rd when acceleration is
needed, including going up hills. In hilly terrain, the trans.
will be constantly shifting back and forth, so most overdrive
automatics provide for a way to either leave the selector in D
(drive, or 3rd) or to switch the OD off.
You also don’t have much engine braking in OD. Engine braking
works like this: if your engine is idling, it’s turning few rpms.
This tends to cause the wheels to turn few rpms too and slows you
down. The lower the gear, the more engine braking because of the
gear ratios. In overdrive, because of its gear ratio, engine
braking isn’t very effective. It’s a good idea to switch off OD or
to select D (or a lower gear) when descending a steep hill to take
advantage of engine braking.
- Hide quoted text — Show quoted text -
In article <2usa3k$…@galaxy.ucr.edu>, David Lu <l…@ucrengr.ucr.edu> wrote:
>First, let me admit that I don’t know much about cars.
>I know to step on the gas and turn the steering wheel,
>that’s about it.
>I just bought an Altima that has an "Overdrive" with
>the automatic. What is this for? All the salesman
>can tell me is "use overdrive on the freeway, other-
>wise turn it off". But the manual says to "use
>overdrive during normal driving conditions, turn it
>off for going up/down steep hills" (paraphrased).
>So what is overdrive? does it cause the transmission
>to switch to a higher gear quickly than if overdrive
>is not used? if so, then why is overdrive necessary
>when there is already a first and second gear on the
>automatic for steep driving?
>i tend to believe that i should follow the manual’s
>advice, rather than the salesman’s. is this correct?
>the manual also mentioned something about "engine
>braking". what is it? thanks for the replies. i
>know we have some auto wizards on this group.
> - lulu
>–
>—==l…@cs.ucr.edu==— just another overworked graduate student.
>David T Lu, Amateur Thinker: l…@cs.ucr.edu, {ucsd, uci}!cs!lulu
>"I seem to be having tremendous difficulty with my lifestyle."
> - Arthur Dent, Hitchhiker’s
David,
In its purist sense "overdrive" is an gear ratio that causes the engine
to turn less rpms that the transmission output shaft. Historically,
back in the early days most cars had 3 speed manual trasmissions. Some
more sporting cars, in order to get higher speeds/lower rpms/etc. added
external overdrive units to end of the transmission. This was
essentially an extra mini-transmission that with the flick of a switch
could add an extra gear chan.
These were kind of klunky and maybe trouble prone. In the ’60s and into
the ’70s, these add-on units faded away in favor of 4 speed
transmissions. Most of the early four speeds were created for
performance reasons with closer spacing in the gear ratios (that 3 speed
boxes) to keep the engines in their power bands. Yet most still had top
gear as a 1:1 ratio.
As fuel economy became more important than performance and due to
pressue from imports, in the late ’70 and into the present you started
seeing more and more 5 speed boxes, where 5th gear was an overdrive
gear. This ment that the gear ration was <1:1. The net result of this
was lower engine rpms at highway speeds and thus higher fule economy,
and thus better CAFE numbers. Today you are starting to see more and
more 6 speed boxes.
In the ’80s and on you started seeing (due to the same pressures) this
approach being applied to automatic transmissions. It again started
with the imports and eventually was adopted by the domestic
manufactures. In the automatic universe there were two main features.
The first was the 4 speed automatic overdrive and the second was the
lockup torqe converter. The 4 speed automatic added an extra gear to
the traditional 3 speed auto. I was usually an overdrive <1:1 ratio.
In conjunction with this was the lockup torqconverter. This was a
feature that at crusing speeds would direct couple the engine to the
transmission (like a manual transmission) thus eliminating the
inefficiencies of fluid coupling. Again this was mostly done for the
sake of fuel economy, but it also gives a smoother/quieter highway ride.
In your case you have a 4 speed overdrive automatic. You can manually
select any of the four gears. It does not have anything to do with how
fast it upshifts it just provides an extra gear for better economy. The
best reccomendation is for normal driving just put in overdrive and
don’t worry about it. What the sales people are confusing you about is
that if you are in a traffic situation where you going to be
consistantly at the shift threashold between 3rd and 4th for (usually 35
to 45 mph) for an extended period of time, you might want to shift to
3rd (turn off overdrive) to keep the transmission from seeking up and
down between the gears (tends to be a little hard on the lockup clutch).
Regarding engine breaking, that is where you shift to a lower gear when
descending a steep grade to let the engine compression help slow you
down and keep the speed reasonable rather than riding the brakes the
whole way.
Sorry for the rambling, hope this helps.
E. R. Sterbenz
e…@cbnmva.att.com
e…@unix.cb.att.com (E.R. Sterbenz) writes:
>In its purist sense "overdrive" is an gear ratio that causes the engine
>to turn less rpms that the transmission output shaft.
[And continues with a thorough explanation of various means by which
this is accomplished.]
I understand the technical definition of "overdrive". But I’m still
curious *why* anyone cares whether the driveshaft turns faster than
the engine. The vehicle speed for a given engine speed is also
influenced by the gear ratio in the differential and by the diameter
of the tires. The bottom line (for performance, fuel economy, etc.)
is engine revolutions per mile, is it not?
Nothing magical occurs at the 1:1 transmission ratio. So why is an
"overdrive" transmission (with ratio less than 1:1) considered
something special? This distinction seems especially silly in FWD
designs, where the transmission and differential are integrated and
the "driveshaft" between them never leaves the transmission housing.
—
Ed Taft t…@adobe.com
In article <2usa3k$…@galaxy.ucr.edu> l…@ucrengr.ucr.edu (David Lu)
writes:
- Hide quoted text — Show quoted text -
> First, let me admit that I don’t know much about cars.
> I know to step on the gas and turn the steering wheel,
> that’s about it.
> I just bought an Altima that has an "Overdrive" with
> the automatic. What is this for? All the salesman
> can tell me is "use overdrive on the freeway, other-
> wise turn it off". But the manual says to "use
> overdrive during normal driving conditions, turn it
> off for going up/down steep hills" (paraphrased).
> So what is overdrive? does it cause the transmission
> to switch to a higher gear quickly than if overdrive
> is not used? if so, then why is overdrive necessary
> when there is already a first and second gear on the
> automatic for steep driving?
> i tend to believe that i should follow the manual’s
> advice, rather than the salesman’s. is this correct?
> the manual also mentioned something about "engine
> braking". what is it? thanks for the replies. i
> know we have some auto wizards on this group.
> - lulu
> —
> —==l…@cs.ucr.edu==— just another overworked graduate student.
> David T Lu, Amateur Thinker: l…@cs.ucr.edu, {ucsd, uci}!cs!lulu
> "I seem to be having tremendous difficulty with my lifestyle."
> - Arthur Dent, Hitchhiker’s
Well, since you’re not knowledgable on the subject of cars, I’ll tell you
simply to put it in overdrive and keep it there. In town, on snow, on the
highway. All it is is 4th gear. It is called overdrive because the gear
ratio is higher. If you want to slow down A LOT on icy roads, them put it
in 1 or 2. If you accelerate and need more power, don’t worry. The car
will shift itself. If you had a three speed auto (or a four speed stick)
would you put it in 2nd gear or 3rd gear to drive? 3rd! (3rd or 4th in a
stick? 4th!). Overdrive is more fuel efficient, and will cause less
engine braking (see other post titled Re:overdrive?). There are special
cases when you will want to leave it off, but they’re rare. Ex: There is
a big hill (about 6 or 8 miles long) near my home town, and (under full
throttle the whole time) the car will downshift to get more power at 65
MPH. The hill is pretty steep, and after ten or so seconds, I’ll hit 73
or 74, and the computer will upshift to 4th (OD). Well, that causes me to
slow down, and I hit 65 and upshift again. Incredibly annoying, so I take
OD off and just go. Just don’t let your revs go over redline.
Mike
This is a followup question:
My 84 Camry failed to start several days ago. It was hot and humid that day.
The engine started at the second try. Later, I found out that the overdrive had
been accidentally set on before starting. Does anyone know if this is the reason
for the engine failure? Everything else seems to be ok.
Thanks for any advice.
–Linda
In article <2ut21m$…@mustang.alleg.edu>, rom…@bio5.alleg.edu (Michael Romeo) writes:
|> Well, since you’re not knowledgable on the subject of cars, I’ll tell you
|> simply to put it in overdrive and keep it there.
Even when starting the engine?
In article <1994Jun29.215349.22…@adobe.com>,
Ed Taft <t…@mv.us.adobe.com> wrote:
>Nothing magical occurs at the 1:1 transmission ratio. So why is an
Nothing magical, that’s true, but the 1:1 ratio is different.
At 1:1 the input and output shaft are directly coupled without
going through the driven shaft. Or at least that’s the way most
manuals worked last time I looked at one.
—
David J. Heisterberg And you all know security
Department of Chemistry is mortals’ chiefest enemy.
The Ohio State University — Shakespeare’s _Macbeth_
In a previous article, l…@ucrengr.ucr.edu (David Lu) says:
>I just bought an Altima that has an "Overdrive" with
>the automatic. What is this for? All the salesman
>can tell me is "use overdrive on the freeway, other-
>wise turn it off". But the manual says to "use
>overdrive during normal driving conditions, turn it
>off for going up/down steep hills" (paraphrased).
I’ll add to what many others have already said.
Concerning the advice of teh salesman- most likely, he
wishes you to be impressed with your purchase. In blvd traffic,
say 35-45mph, leaving the car out of overdrive will result
in peppier driving. The car will have more power in 3rd, as
its at a higher engine Rev. I’ve noticed, while testing
driving vehicles lately, the salesman instructs me to shit
it into 3d, and not overdrive. =)
That aside, if the Altima has more than enough power for
your driving habits, I’ld leave the overdrive on. I tend
to leave it on. Then flick it off before I know I’m about to
accelerate, e.g. doing 40 and about to hit an expressway
onramp. I prefer the autos with the overdrive control as
a button on the shift lever, rather than as a D4.
—
So there I was, snuggled in the leather seat of my brand new Ferrari.
I had the oiled wooden gear shift in one hand, and the leather wrapped
Momo in the other. I had the stereo cranking _Born to be Wild_…
Only two problems, I was upside down, and under six feet of water…
In <2usa3k$…@galaxy.ucr.edu> l…@ucrengr.ucr.edu (David Lu) writes:
>First, let me admit that I don’t know much about cars.
>I know to step on the gas and turn the steering wheel,
>that’s about it.
>I just bought an Altima that has an "Overdrive" with
>the automatic. What is this for? All the salesman
>can tell me is "use overdrive on the freeway, other-
>wise turn it off". But the manual says to "use
>overdrive during normal driving conditions, turn it
>off for going up/down steep hills" (paraphrased).
>So what is overdrive? does it cause the transmission
>to switch to a higher gear quickly than if overdrive
>is not used? if so, then why is overdrive necessary
>when there is already a first and second gear on the
>automatic for steep driving?
Overdrive is a fourth gear on most automatics with a
low ratio of gearing. The idea is that when your car
is up to speed, the overdrive allows the engine to
turn fewer revolutions per mile saving gas and wear
on the engine.
>i tend to believe that i should follow the manual’s
>advice, rather than the salesman’s. is this correct?
Yes. If it is an automatic it will shift down to third
gear whenever the engine does not have enough torque.
If you really are in middle city where you seldom get
above 30 mph it probably makes sense not to use the
overdrive.
>the manual also mentioned something about "engine
>braking". what is it? thanks for the replies. i
>know we have some auto wizards on this group.
Engine braking is when you take your foot off the gas
pedal and the engine slows down, acting like a brake.
Because of the low gear ratio overdrives (or 4th gears)
do not offer much of this braking effect. My Probe has
an overdrive automatic and there is no noticeable
braking effect in overdrive.
> - lulu
>–
>—==l…@cs.ucr.edu==— just another overworked graduate student.
>David T Lu, Amateur Thinker: l…@cs.ucr.edu, {ucsd, uci}!cs!lulu
>"I seem to be having tremendous difficulty with my lifestyle."
> - Arthur Dent, Hitchhiker’s
Bob B.
In article <1994Jun29.215349.22…@adobe.com> t…@mv.us.adobe.com (Ed Taft) writes:
}
}I understand the technical definition of "overdrive". But I’m still
}curious *why* anyone cares whether the driveshaft turns faster than
}the engine. The vehicle speed for a given engine speed is also
}influenced by the gear ratio in the differential and by the diameter
}of the tires. The bottom line (for performance, fuel economy, etc.)
}is engine revolutions per mile, is it not?
}
}Nothing magical occurs at the 1:1 transmission ratio. So why is an
}"overdrive" transmission (with ratio less than 1:1) considered
}something special? This distinction seems especially silly in FWD
}designs, where the transmission and differential are integrated and
}the "driveshaft" between them never leaves the transmission housing.
1) Historical reasons (see other posts concerning separate overdrive units)
2) Marketing Marketing Marketing.
–
Matthew T. Russotto russo…@eng.umd.edu
Some news readers expect "Disclaimer:" here.
Just say NO to police searches and seizures. Make them use force.
(not responsible for bodily harm resulting from following above advice)
In article 5…@zippo.uwasa.fi, Wo…@freeport.uwasa.fi (Keith Ryan) writes:
|>
|>In a previous article, l…@ucrengr.ucr.edu (David Lu) says:
|>
|>>I just bought an Altima that has an "Overdrive" with
|>>the automatic. What is this for? All the salesman
|>>can tell me is "use overdrive on the freeway, other-
|>>wise turn it off". But the manual says to "use
|>>overdrive during normal driving conditions, turn it
|>>off for going up/down steep hills" (paraphrased).
|>
|> I’ll add to what many others have already said.
|>Concerning the advice of teh salesman- most likely, he
|>wishes you to be impressed with your purchase. In blvd traffic,
|>say 35-45mph, leaving the car out of overdrive will result
|>in peppier driving. The car will have more power in 3rd, as
|>its at a higher engine Rev. I’ve noticed, while testing
|>driving vehicles lately, the salesman instructs me to shit
^^^^ ?
(:-) is this your opinion of the Altima? (-:)
|>it into 3d, and not overdrive. =)
Since the discussion was about an automatic transmission,
There is no reason not to use the OverDrive setting for
normal driving. At 35-45 mph, it probably won’t be
in the OD gear anyway. For starting up for a full stop,
it will sequence through the lower gears in the same
way no matter whether it is in D or OD.
The only reasons for putting an overdrive automatic in D
rather than OD is if you are pulling a trailer or driving
in hilly areas where the transmission seems to be "hunting" –
that it frequently shifting up and down.
—
Robert Haar InterNet : rh…@gmr.com
Computer Science Dept., G.M. R & D Center
DISCLAIMER: Unless indicated otherwise, everything in this note is
personal opinion, not an official statement of General Motors Corp.
In article <2v10fl$…@mojo.eng.umd.edu>,
Matthew T. Russotto <russo…@eng.umd.edu> wrote:
>In article <1994Jun29.215349.22…@adobe.com> t…@mv.us.adobe.com (Ed Taft) writes:
>}
> *why* anyone cares whether the driveshaft turns faster than
So why have a third gear?
So why have a second gear?
So you can drive the car!
To optimize the power band of the engine.
For example, my van had a 3 speed with 2.73 rear end. It was a dog.
I changed the rear gear to 3.23 and the tranny to an over drive.
The overdrive has a deeper first gear, and a taller fourth gear.
So now I have about the same rev’s in top gear on the hiway (my
mpg is the same) and deeper gear …read moe go… every where
else which improved mpg in the city since the engine is not laboring.
Its not that I care about the drive shaft itself, I care about
power and economy.
___Now, My question is… what in the world has towing got to do
with over drive????
If I’ve got 4.11 gears, chances are I will be towing in overdrive
at 60 mph, if I didn’t use over drive while towing, I would be
buzzing the motor at 3300+rpm with the 4.11′s.
John, Cuyahoga Valley Vans, member, Northern Ohio Van Concil
In article <2usa3k$…@galaxy.ucr.edu> l…@ucrengr.ucr.edu (David Lu) writes:
>i tend to believe that i should follow the manual’s
>advice, rather than the salesman’s. is this correct?
This is correct not matter what the situation. All salesman care
about is selling. Don’t trust anything they say about anything
without checking it out first.
Of course this doesn’t necessarily apply to all salesmen. Just
the ones I have dealt with. <g>
Keith Winfree <tcwi…@nvmusic.vccs.edu>
’68 Charger 383 (1 of 19)
’70 Charger 500 318
In article <2v16qf$…@sulawesi.lerc.nasa.gov> yy…@jzola.lerc.nasa.gov (John Burkett) writes:
>For example, my van had a 3 speed with 2.73 rear end. It was a dog.
>I changed the rear gear to 3.23 and the tranny to an over drive.
>The overdrive has a deeper first gear, and a taller fourth gear.
>So now I have about the same rev’s in top gear on the hiway (my
>mpg is the same) and deeper gear …read moe go… every where
>else which improved mpg in the city since the engine is not laboring.
And a very good reason. But also there are limitations to overdrive, the
biggest being that it is a geared ratio and subject to wear/drag. Not much
you can do about it with a transaxle since all ratios are geared, but the
1:1 direct drive in other transmissions is a simple lockup of the mainshaft.
The gears are still spinning but have no load on them & last much longer.
This is part of the reason why GM recommends leaving a 700R4 automatic
overdrive in "D" when pulling a trailer.
Now if it is cheaper to go from a four-speed to a five-speed o/d or you have
some other reason, fine but it is always better to be in a direct gear most
of the time. Overdrives are attractive because it is really inexpensive for
a manufacturer to offer as an option.
Years ago you could buy most manual gearboxes with either close or wide
ratios (only difference was number of teeth on the input gear and the
meshing surface on the cluster gear) so that a wide box with a 3.23 axle
could have the same 1-2-3 as a close box with a 3.90 & have a nice highway
ratio in fourth. In either case, 4th was direct and had no geared losses or
thrust loading on the box.
Today, manufacturers do not offer your choice of 10 or more axle ratios,
instead 2 or 3 is more likely so OD is a popular "extra cost option" but it
is not necessarily the best engineering choice. Of course they are usually
well out of warranty before they start to howl.
A. Padgett Peterson, P.E.
Cybernetic Psychophysicist
We also walk dogs
PGP 2.4 Public Key Available
I have a question, how much does one extra gear cost? (perhaps mechanically
it gets much more complicated, but on mass produced cars, I dont’ see it would
be that much…) How much do you think it’d cost? Perhaps provide it as
an option for $500 or so??
Wouldn’t majority of today’s "sporty" cars, ie cars and does 0-60 in about
6.5-9 seconds, quarter mile in about 14.5-16 seconds benefit greatly from
closer ratio 5 speed gear box, plus an 6th overdrive?
–
Frank M. Lin
fm…@netcom.com
In a previous article, fm…@netcom.com (Frank M. Lin) says:
>I have a question, how much does one extra gear cost? (perhaps mechanically
>it gets much more complicated, but on mass produced cars, I dont’ see it would
>be that much…) How much do you think it’d cost? Perhaps provide it as
>an option for $500 or so??
Considering, to add an complete automatic transmission, with
overdrive, costs on the order of $500-900 as an option; I believe your
estimate is a little high. Also, selling more cars with an overdrive,
raises the total fuel milage ratings for the manufactors. ( C.A.F.E. )
Thereby, lowering any penalties to the company, and hopefully, should
see as a decrease in the selling price of the car.
>Wouldn’t majority of today’s "sporty" cars, ie cars and does 0-60 in about
>6.5-9 seconds, quarter mile in about 14.5-16 seconds benefit greatly from
>closer ratio 5 speed gear box, plus an 6th overdrive?
I’ld like to see this both on manual and automatic. Lower
first and second gears for performance, with a high overdrive for
economy. ( as is, most cars attain their highest speed in 3rd ( auto)
5th (manual ), and not the highest gear avaliable ).
—
So there I was, snuggled in the leather seat of my brand new Ferrari.
I had the oiled wooden gear shift in one hand, and the leather wrapped
Momo in the other. I had the stereo cranking _Born to be Wild_…
Only two problems, I was upside down, and under six feet of water…