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.
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.
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.
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.
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.
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.
Yes, they did have freewheeling. IIRC, there was a lever on the side of the bellhousing tranny area where it could be locked out.
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.
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.
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. :eek:
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.
Forgot to add, at motorway cruising speeds (60-70mph here in the UK) the RPMs dropped by about 500rpm with O/D.
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.
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.
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.