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  #1  
Old 02-27-2006, 02:07 AM
pool pool is offline
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How/why do you use Overdrive on car?

I know overdrive has something to do with fuel efficiency, but under what conditions and how do you use it.
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  #2  
Old 02-27-2006, 06:58 AM
TheLoadedDog TheLoadedDog is offline
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Quote:
Originally Posted by pool
I know overdrive has something to do with fuel efficiency, but under what conditions and how do you use it.
Please allow me to introduce you to a thread full of contradictory opinions (but it's better than nothing) that I started on this subject a while back.
http://boards.straightdope.com/sdmb/...d.php?t=235555
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  #3  
Old 02-27-2006, 09:50 AM
Dorjän Dorjän is online now
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In my car at least (and I suspect most vehicles) overdrive is simply a 4th gear. With overdrive on, the car will shift into 4th gear at higher speeds and kind of "coast" along with it. It does increase fuel efficiency, although by how much I can't say. Turning overdive off restricts the car to 3rd gear. In my car (a Mazda Protege) RPMs at 65 mph are around 3200. With overdrive off and restricted to 3rd gear, it's 4500. It doesn't hurt the car or anything, it's just less efficient. When towing a trailer, I have to have overdrive off to prevent the car from shifting into 4th gear, because the least amount of towing power is available and I'd run the risk of overworking the engine and transmission.
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Old 02-27-2006, 10:42 AM
Gary T Gary T is offline
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A better question is in what conditions do you not use it.

The vehicle is designed to use the overdrive gear when conditions warrant. The transmission (I'm assuming an automatic) will shift into OD when appropriate, just as it shifts into 1st, 2nd, & 3rd when appropriate. There's one shortcoming in most designs in that in some hilly terrain, there will be continuous shifting back and forth between 3rd & 4th (OD). This is called "hunting," and it's not desired. The reason there's an OD switch is so that you can disable OD in this situation. Owner's manuals typically spell this out.

Transmission specialists tell me that some transmissions will fare better if OD is also disabled when towing a heavy load.

Other than those two situations - hunting on hills, heavy towing - there's really no good reason to disable overdrive. Overdrive allows the engine to run at a lower speed, which improves gas mileage and longevity.
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  #5  
Old 02-27-2006, 12:20 PM
Mr. Slant Mr.  Slant is offline
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At 55 MPH in a 2002 Mercury Sable with Ford's 3.0L 24V V6, informal testing has shown fuel economy differences in the 10-20% range.
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  #6  
Old 02-27-2006, 12:25 PM
Mr. Slant Mr.  Slant is offline
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Quote:
Originally Posted by Gary T
SNIP
Other than those two situations - hunting on hills, heavy towing - there's really no good reason to disable overdrive. Overdrive allows the engine to run at a lower speed, which improves gas mileage and longevity.
That, and maybe engine braking on long, steep hills. In my old Mercury it meant the difference between coasting down a given hill (mountain, really) at 72 and coasting down the same hill at speeds that would likely have approached 100 MPH. Did a good job of saving my brakes in West Virginia.
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  #7  
Old 02-27-2006, 01:10 PM
Gary T Gary T is offline
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Quote:
Originally Posted by Mr. Slant
That, and maybe engine braking on long, steep hills. In my old Mercury it meant the difference between coasting down a given hill (mountain, really) at 72 and coasting down the same hill at speeds that would likely have approached 100 MPH. Did a good job of saving my brakes in West Virginia.
Good point.
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  #8  
Old 02-27-2006, 01:46 PM
ralph124c ralph124c is offline
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Didn't the old two-stroke SAAB's have freewheeling? The engine would actually decouple from the transmission when descending a hill-that was done to avoid scoring the cylinder walls ( a two stroke engine depends on the oil/gasoline mixtur to lubricate the cylinder walls. I just wonder of modern auto tarnnies use some kind of freewheeling mode as well. I'm GLAD I drive a manual tranny car-not only is the gas mileage better, my brakes last upwards to 80,000 miles between pad replacements.
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  #9  
Old 02-27-2006, 02:31 PM
GaryM GaryM is offline
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Yes, they did have freewheeling. IIRC, there was a lever on the side of the bellhousing tranny area where it could be locked out.
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  #10  
Old 02-27-2006, 04:44 PM
Mr. Slant Mr.  Slant is offline
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Quote:
Originally Posted by ralph124c
SNIP I just wonder of modern auto tarnnies use some kind of freewheeling mode as well. SNIP
Scene: I'm tooling along at 55 in my Sable with the cruise on. RPMs are just under 2000 RPM in overdrive. The terrain goes from flat to a slight downhill.
My car is under zero throttle pressure. The RPM drops to just under 1500 RPM, but I continue accelerating.
Was that freewheeling?
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  #11  
Old 02-27-2006, 07:07 PM
Rick Rick is offline
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Quote:
Originally Posted by Mr. Slant
Scene: I'm tooling along at 55 in my Sable with the cruise on. RPMs are just under 2000 RPM in overdrive. The terrain goes from flat to a slight downhill.
My car is under zero throttle pressure. The RPM drops to just under 1500 RPM, but I continue accelerating.
Was that freewheeling?
No.
Freewheeling would be if your engine had returned to idle and the car continued to accelerate. The same as if you had shifted into neutral.
Picture if you will a manual transmission car under the conditions described, cruise, and then a downhill. If you did not take the car out of gear, it would accelerate (assuming that the downhill was steep enough) even though it was still in gear. If however, you pushed the lever into neutral, the cruise would disengage, the engine would return to idle, and the car would speed up more than before as it no longer had the drag of the engine holding it back. This is freewheeling. Well not exactly, but close enough for this discussion.
On an automatic transmission none of the gears freewheel, but some of them (Not the highest) give engine braking. If you have ever moved your shift lever down from D to S or 4 or 3 or 2 or D2 or whatever it is called in your car you have felt engine braking when you took your foot off the gas.
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  #12  
Old 02-27-2006, 10:08 PM
Kiminy Kiminy is offline
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I have a 2000 Mazda MPV minivan, that uses "overdrive" as its default setting. (You have to push a button to turn it off, and a light comes on to let you know it's been turned off.)

Unfortunately, this is also the first automatic transmission vehicle I have owned. All my other cars have been stick-shift, which allowed me to choose the appropriate gear for whatever terrain I happened to be driving in. (My fave--putting the '67 VW bug in neutral for the slight grade down I-26 from Hendersonville NC to Spartenburg SC. Just enough of a slope to keep the car going between 55 and 60 (back when 55 was the speed limit everywhere in the country) without using the gas pedal, brakes, or non-existant cruise control!)

I now live in the relative "flat lands" of Louisville, Kentucky, where we have very few steep inclines. When I visited my father in Western NC a few months after buying this car, though, I found it *extremely* difficult to get to the top of the mountain where he lived, and I suspect it's because my car was in Overdrive, but I didn't know the difference. He moved to California shortly thereafter, and my mother is quite perched on such a steep hill, so I haven't had any problems otherwise.
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  #13  
Old 02-28-2006, 02:11 AM
DarrenS DarrenS is offline
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CarTalk's take on the subject.

/Aside: are "Click and Clack" considered credible authorities among automotive experts?
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  #14  
Old 02-28-2006, 06:49 AM
flex727 flex727 is offline
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Quote:
Originally Posted by DarrenS
CarTalk's take on the subject.

/Aside: are "Click and Clack" considered credible authorities among automotive experts?
In my opinion, they're credible. Now you have to ask yourself if I'm credible.

Okay, sorry, now you're worse off than you were before I responded.
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  #15  
Old 02-28-2006, 07:17 AM
Pushkin Pushkin is offline
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Quote:
Originally Posted by ralph124c
Didn't the old two-stroke SAAB's have freewheeling?
I vaguely remember something about a flywheel on the engine to continue lubrication that could also be found on the four stroke Rover P6.

My Dad's old Volvo 740 had overdrive on the manual gearbox. It was another gearbox between the gearbox and the rear axle, easy to fit on a RWD car. Activation was by a button ontop of the gearlever, the mechanism was similar to that of an autobox so a single press of the button was all that was required. Dad didn't beleive this and insisted on pushing in the clutch pedal at the same time.
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  #16  
Old 02-28-2006, 07:19 AM
Pushkin Pushkin is offline
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Forgot to add, at motorway cruising speeds (60-70mph here in the UK) the RPMs dropped by about 500rpm with O/D.
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  #17  
Old 02-28-2006, 08:32 AM
Rick Rick is offline
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Quote:
Originally Posted by DarrenS
CarTalk's take on the subject.

/Aside: are "Click and Clack" considered credible authorities among automotive experts?
IMHO no.
They are great entertainment, but every time I listen to them, I wind up screaming at the radio as they go for the joke, not correct information.
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  #18  
Old 02-28-2006, 09:08 AM
Sunspace Sunspace is online now
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If overdrive is just an optional top-end gear-ratio, why is it given a special name?

When I was younger, I always thought overdrive was some kind of attachment that gave a burst of extra power on demand. I know exactly where I got this impression: Scooby Doo cartoons. At one point the gang are running from someone in the Mystery Machine, and they put the van into overdrive and it suddenly accelerates, and they escape.
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  #19  
Old 02-28-2006, 09:55 AM
Gary T Gary T is offline
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Quote:
Originally Posted by Sunspace
If overdrive is just an optional top-end gear-ratio, why is it given a special name?

When I was younger, I always thought overdrive was some kind of attachment that gave a burst of extra power on demand. I know exactly where I got this impression: Scooby Doo cartoons. At one point the gang are running from someone in the Mystery Machine, and they put the van into overdrive and it suddenly accelerates, and they escape.
Well, those Scooby Doo writers should have consulted their Star Wars manual - that sounds like hyperdrive, not overdrive. Overdrive is like having a higher gear on a bicycle. It allows greater ultimate speed, but it doesn't contribute to blazing acceleration.

In traditional manual transmissions (i.e., not transaxles) top gear was always direct drive - the input shaft was connected directly to the output shaft, so that output speed was the same as engine speed. In the lower gears, output speed was less than engine speed. Lower gears give more "leverage" to accelerate from take-off or slow road speeds, higher gears give more ultimate road speed.

So naturally some people wanted to go faster than the above set-up would allow, and one way to do that was to add an overdrive unit - the attachment you mention. It used another gear set to make its output speed greater than its input speed, delivering more speed to the final drive (rear end/differential) thus allowing a higher road speed. This is what some old Volvos and MG's had.

Then came the idea to incorporate this effect into the transmission itself. An additional gear was added that made output speed greater than input speed. The next-to-top gear was still direct drive, but now top gear was overdrive. This was done first in manual transmissions, then in automatice transmissions.

Transaxles can't use direct drive, but they can achieve the same effect (output speed = input speed) through gearing. They can also achieve the effect of overdrive through gearing, and if output speed in top gear is greater than input speed, it's called an overdrive transaxle.

So the special name relates to the history of transmission design and the feature of having output speed exceed input speed.
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  #20  
Old 02-28-2006, 10:34 AM
Pushkin Pushkin is offline
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Quote:
Originally Posted by Sunspace
If overdrive is just an optional top-end gear-ratio, why is it given a special name?
Originally overdrive was a part of the transmission that wasn't just built into the gearbox. IIRC, it was an optional extra that you could fit aftermarket. So being a separate entity it had a separate name.
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  #21  
Old 02-28-2006, 11:14 AM
Sunspace Sunspace is online now
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Quote:
Originally Posted by Gary T
In traditional manual transmissions (i.e., not transaxles) top gear was always direct drive - the input shaft was connected directly to the output shaft, so that output speed was the same as engine speed. In the lower gears, output speed was less than engine speed. Lower gears give more "leverage" to accelerate from take-off or slow road speeds, higher gears give more ultimate road speed.
Okay, that makes sense. You're using 'speed' to meen 'rotational speed', in revolutions per unit time, not the speed of the car over the road, right?
Quote:
So naturally some people wanted to go faster than the above set-up would allow, and one way to do that was to add an overdrive unit - the attachment you mention. It used another gear set to make its output speed greater than its input speed, delivering more speed to the final drive (rear end/differential) thus allowing a higher road speed. This is what some old Volvos and MG's had.

Then came the idea to incorporate this effect into the transmission itself. An additional gear was added that made output speed greater than input speed. The next-to-top gear was still direct drive, but now top gear was overdrive. This was done first in manual transmissions, then in automatice transmissions.

Transaxles can't use direct drive, but they can achieve the same effect (output speed = input speed) through gearing. They can also achieve the effect of overdrive through gearing, and if output speed in top gear is greater than input speed, it's called an overdrive transaxle.

So the special name relates to the history of transmission design and the feature of having output speed exceed input speed.
That's a very clear explanation.

I have an idea of a traditional manual transmission: two sets of gears with a clutch that lets you disconnect the input from the power source, so you can move a different output gear against the input gear. What's a transaxle, then, if it's different than a traditional manual transmission?

I thought a transaxle just combined the gearchange unit of a regular transmission with axles for the drive wheels, and was used to save space in front-wheel-drive front-engine cars, but it didn't actually function any differently?
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  #22  
Old 02-28-2006, 11:24 AM
Rick Rick is offline
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Quote:
Originally Posted by Sunspace
Okay, that makes sense. You're using 'speed' to meen 'rotational speed', in revolutions per unit time, not the speed of the car over the road, right? That's a very clear explanation.
Yes 100 RPM in 100 RPM out = direct drive. 100 RPM in 120 RPM out = overdrive

Quote:
I have an idea of a traditional manual transmission: two sets of gears with a clutch that lets you disconnect the input from the power source, so you can move a different output gear against the input gear. What's a transaxle, then, if it's different than a traditional manual transmission?

I thought a transaxle just combined the gearchange unit of a regular transmission with axles for the drive wheels, and was used to save space in front-wheel-drive front-engine cars, but it didn't actually function any differently?
You got it, combine a transmission with the axle unit and you have a transaxle. Function is the same as a RWD manual transmission. The design is different due to the difference in packaging.
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  #23  
Old 02-28-2006, 11:43 AM
Gary T Gary T is offline
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Quote:
Originally Posted by Sunspace
I thought a transaxle just combined the gearchange unit of a regular transmission with axles for the drive wheels, and was used to save space in front-wheel-drive front-engine cars, but it didn't actually function any differently?
To clarify, it uses a gearset, as does a regular (RWD) transmission, but the gearset is not just like, or "of," a regular transmission. As Rick mentioned, it requires a different layout and thus is designed differently, one difference being it doesn't have direct drive - input shaft directly coupled to output shaft - like a regular transmission.

The "-axle" part of "transaxle" is a reference to the final drive axle of a traditional RWD set-up, which has the ring and pinion and the differential, and usually (but not always) the actual rear axles. It does not refer to the unit containing the actual axles, which are external to it.
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  #24  
Old 02-28-2006, 12:13 PM
Bryan Ekers Bryan Ekers is online now
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Quote:
Originally Posted by Sunspace
At one point the gang are running from someone in the Mystery Machine, and they put the van into overdrive and it suddenly accelerates, and they escape.
They obviously gave the van a Scooby-snack.
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  #25  
Old 02-28-2006, 12:15 PM
saoirse saoirse is offline
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Originally Posted by GaryM
Yes, they did have freewheeling. IIRC, there was a lever on the side of the bellhousing tranny area where it could be locked out.
That's wierd. Which models?
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  #26  
Old 02-28-2006, 02:20 PM
Mr. Slant Mr.  Slant is offline
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Quote:
Originally Posted by Rick
No.
Freewheeling would be if your engine had returned to idle and the car continued to accelerate. The same as if you had shifted into neutral. SNIP
So what was I experiencing when I had the sudden RPM drop in my Sable going downhill?
I was 10 miles into a 13 mile drive on a 60-degree day.
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  #27  
Old 02-28-2006, 08:27 PM
Rick Rick is offline
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::: sigh:::
I was afraid somebody was going to ask me that.
To explain the difference I am going to have to give you a short course in how an auto trans works.
What makes an automatic transmission work is planetary gears . Planetary gears can give different ratios depending on which gear is held stationary, which one gets power and which one is the output. hydraulic pressure is directed to clutches and brakes and freewheels inside the transmission. Clutches lock rotating members together, brakes stop a rotating member, and freewheels (sometimes called sprag clutches, or one way clutches) allow free movement in one direction, but lock up in the other. Think of a ratcheting screwdriver.
Locking any two parts of the planetary gear (clutch) set together makes the entire unit rotate as if it were welded into a solid piece. Brakes (band or disc) stop a shaft from rotating, and freewheels back up the clutches so that the trans does not slip under load.
When the engine is running it spins the torque converter and the oil pump inside the transmission. The torque converter is a fluid coupling takes the place of a clutch on a manual transmission car. (it does some other stuff also, but this is good enough for now) When you are in neutral there is oil pressure in the transmission, but the different solenoids are not energized this means that the clutches, brakes, and freewheels are not applied so the transmission's output shaft is not coupled to the input shaft. The same is true in park, except there is a mechanical lock on the output shaft that is engaged.
When you shift to drive (I am not going to explain the power flow for each gear. That will be left as an exercise for the reader.) the clutches brakes and freewheels for 1st gear are engaged. As you accelerate the transmission shifts from 1 to 2 and so on until you are in your top gear (3, 4, 5, 6 or I think BMW has a 7 speed)
Anyway you hit a downhill grade. It does not matter if your cruise is engaged, or how long you have been driving. We are going to assume a rather steep grade for the sake of this discussion. When you start down this hill, gravity, not only being a good idea, but the law acts on your car and pulls it to the bottom of the hill. Being a steep hill, your car actually speeds up somewhat. If you look closely at the tachometer you will see that your engine is running at say 1700 RPM. Why is that? Simple, the brakes, clutches and freewheels for whatever gear you were in before you got to the hill are still applied. The output shaft of the trans in still coupled to the input shaft. The fluid coupling (torque converter) is working backwards, and now your wheels are driving the engine rather than the engine driving the wheels. Freewheeling is when the engine and the drive wheels are not coupled. Since the drive wheels and the engine are still coupled, this is not freewheeling.
Still not convinced? OK try this on:
Now you are on a very shallow grade. Say 1%. You let off on the gas in gear, and the car does not speed up. Maybe it actually slows down. Why is this? We know gravity is the law and not subject to repeal so that isn't it. The reason is that the output shaft of the trans is still coupled to the input, and the slope is not steep enough to overcome the drag of the engine. If you were to shift to neutral, the car would coast down the hill normally.
If you shift an automatic trans to neutral it will freewheel, leaving it in gear is similar, but is not freewheeling.
It should be noted that shifting a car to neutral on a downgrade is against the law in many places.
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  #28  
Old 02-28-2006, 08:33 PM
Rick Rick is offline
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It just occurred to me after posting that opus, that I did not directly answer your question.
The cruise control had gravity helping it. Therefore the throttle had to close down or the car would have accelerated down the hill. Just as you lift your foot off the gas when heading down a hill, the cruise control also closes the throttle. So the sudden RPM drop was from the throttle closing. The reason the engine did not return to idle was explained in my last post.
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