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Old 01-18-2012, 04:46 PM
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Join Date: Dec 2005
Location: SE Michigan
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Does turning off overdrive improve fuel consumption?

A generally car-savvy friend tells me that turning the OD off for around-town driving will improve economy. The vehicle in question is a 2001 Mercury Villager minivan.

It's my understanding that OD is like fifth gear in a standard transmission vehicle, so there's really no reason to turn it off except maybe when towing or climbing long grades or any time generally when you want to stay in lower gears.

What's the real deal? Thanks.
Old 01-18-2012, 04:47 PM
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No expert, but I thought the whole point of OD was to improve fuel economy.
Old 01-18-2012, 05:08 PM
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Join Date: Apr 1999
Posts: 23,338
I happened to have owned a Mercury Villager, but the type of vehicle is not important.

Turning off the OD is sometimes helpful in city driving when your speed is right around the OD shift point. This doesn't have anything to do with gas mileage, but because some transmissions have poor acceleration in OD, and attempting to speed up just causes them to drop down a gear.

Here's the test. Get your speed up to where you just shift into OD. Then drive normally. If you're constantly dropping out of OD, try doing the same thing with the OD turned off. If you like it better that way, then it's okay to turn off OD. If it doesn't matter, just leave the OD on.
Old 01-18-2012, 07:02 PM
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Join Date: Apr 2010
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Probably no difference

Overdrive probably won't kick in much at all in city driving so it won't make a mileage difference one way or another. However, Ford used to recommend not using the OD unless one were one the highway because frequent up and down shifting was causing premature and expensive transmission failure. Therefore, I would still follow that advice now even if they are not recommending it.
Old 01-18-2012, 10:56 PM
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Join Date: Mar 1999
Location: N/W Arkansas
Posts: 8,398
This is like driving with the tailgate down on a pick-up..... It depends....

Manny peoples have done manny tests and so there is a lot of fighting because it is not one size fits all.

Depending on many factors, there is a speed & load and throttle position that will burn more fuel maintaining a certain speed in OD than it does one step down on certain vehicles with certain transmissions & with certain engines & loads.

So at that particular speed of say 62 miles per hour & all those conditions your millage with the OD is say 16.6 MPG at 2100 RPM and X throttle position.

And you could find that with everything else being the same with just a step down in gearing to 2575 RPM and the throttle position to hold everything the same is actually using less fuel so your MPG is better.

In small aircraft with constant speed propellers where many things can be maintained in a constant condition and with a good fuel flow meter system this can be demonstrated quite easily.

In cars & such, it is most noticable when the conditions are such that you can hold speed without quite getting a shift down but you are holding a lot of throttle and where if you drop a gear and the engine runs more free so to speak, that your actual fuel flow will be better.

Less wear & tear on the transmission is the real advantage as was stated up thread.

I do not care what has been proven by Myth Busters and the like, I know what my vehicles do and the fuel burn proves it.

YMMV & all that....

Be consistent over the same road with the same vehicle and do your own controlled, as best you can, test and see if it makes you any difference. If not, tell all those that disagree they are full of it and if you see that you do get a difference, be happy in knowing what you know and do not waste too much time arguing about it.

Do not type up a bunch of explanations and what if's like this as it is truly a waste of time....

BAwhahahaha

I am bored....
So there..........
Old 01-18-2012, 11:19 PM
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Join Date: Aug 1999
Posts: 16,451
In general other factors being equal the lower the RPM the better the gas mileage. This is the reason modern transmissions have more and more gears. ( My first car had a 2 speed auto there was just an ad on TV for an available 8 speed in Chrysler products. FWIW I did a test once on this very subject. I drove the same course using the cruise control once in drive, once in the next gear down, and once in the next gear down from there, while observing the instantaneous fuel mileage. The result? The higher the gear the better the mileage.
With the electronic controls that are on modern cars chances are the computer will pick the correct gear is a lot higher than the chance you will.
Unless I had a super heavy load in the car or I was towing a trailer I would leave the OD on.
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Old 01-20-2012, 05:01 AM
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Overdrive is defined as being a very high gear such that the output shaft of the transmission actually spins faster than its input shaft. Consequently its purpose is to squeeze the maximum fuel economy during times when the lowest amount of torque is required, i.e. highway driving at a constant, reasonably high speed over mostly flat road. Your car's transmission is designed to only use overdrive when it's best suited to. But because it's a bit of a fine line sometimes, they almost always give the driver the option to turn it off via a button on the shifter.

But unless your transmission shifts into it much too early or too late (IOW there's something wrong with it) driving around with it always turned off can do nothing but decrease your fuel economy (though only by a little).

BTW with a standard transmission fifth gear is always overdrive. Automatics use planetary gearing and because planetary gears have three main components automatic transmissions are always inherently three speed. Overdrive on these is usually achieved via an electronically controlled extra gearset on the output shaft (still contained inside the transmission case itself).
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