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Unread 01-26-2010, 10:51 AM   #1
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Default What happens when OD is off?

I have a Nissan Pathfinder 2001 AWD and I do not understand what happens with I turn OD off.

One source says it runs my car at higher RPMs, thus make gas mileage worse, and another source says the total opposite.

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Unread 01-26-2010, 11:00 AM   #2
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the first source is correct. OD spins the wheels faster than a 1:1 ratio to the engine, making it so the engine can run at lower RPMs while coasting at highway speeds.

in my car 3rd gear is 1:1 ratio, then OD kicks in and the RPMs go down. It's generally a good idea to turn OD off if you're racing or having fun with the car lol. in my car a Wide open throttle 3rd to OD shift can ruin the transmission.

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Unread 01-26-2010, 11:06 AM   #3
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Huh? I am confused.

The first source says it runs at higher RPMs, but you're saying it runs at lower RPMs.

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Unread 01-26-2010, 11:25 AM   #4
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sorry i'm confused about your question i guess.

OD on = lower RPMs

OD off = higher RPMs

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Unread 01-26-2010, 11:59 AM   #5
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OD lowers RPM and saves you gas. YOu should keep it on all the time. The car will feel more sluggish in terms of acceleration. But at wide open throttle, there is no difference.
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Unread 01-26-2010, 01:28 PM   #6
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Thanks guys.

EDIT:

I've read from a few sources that is recommended to turn off OD when you're driving in the city where there are a lot street lights and traffic, and also at a constant speed on the highway. I am guessing they are wrong, yes?

Last edited by TaiDinh; 01-26-2010 at 01:38 PM.

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Unread 01-26-2010, 02:47 PM   #7
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It may be better for your transmission to turn it off when not using it, probably not enough that you will notice. The car manufacturers made the transmission expecting that you would leave OD on, because that is the default.

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Unread 01-26-2010, 05:04 PM   #8
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Turning OD off when at a constant highway speed is dumb as hell, Tai. Whoever suggested it needs to get off the highways, they're too dumb to drive. That's the very operating mode Overdrive was designed for. Drops the RPM, locks up the torque converter on automatics, and gives you 3-5MPG. Turning it off would only be advantageous if you are towing a large weight.

There's actually a sticker on my driver's side visor with a bunch of things showing how you should operate the overdrive, both manual and automatic. Every time it shows an unloaded truck, it shows the transmission in OD.
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Unread 01-26-2010, 05:30 PM   #9
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The gears in the automatic transmission do not slip, however, their is a coupling devise that allows you to come to a stop while in gear. This is called the torque converter. Their slippage built into this, but once you are underway, and up to highway speeds this slippage is undesirable. The added slippage increase engine RPM, increasing fuel consumption and emissions, and makes the car noisier and increases wear & tear.

The manufacturers have for quite sometime have used lockup converters to stop all slippage in the transmission. Their is a clutch in the converter that the car's computer locks up when it deems it is "best" for most situations. However, many vehicles, depending on gearing, may subject to "lugging" the engine. Reducing the engine RPM too much can cause increased wear on the engine and if the transmission is constantly shifting in and out of OD (locking the converter), it increases the wear on the transmission.

If you are driving around town, it is OK to turn off the OD. If you see the transmission shifting into OD and then dropping out with just a light touch of the throttle, it's an indication that you would likely be better off just taking it out of OD until you are going a bit faster. This is very common on under powered vehicles and vehicles with tall, or highway gears. Some manufacturers do a real good job of getting this bang on and it's not necessary to shift out of OD, others do a very poor job.
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Unread 01-26-2010, 05:50 PM   #10
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I feel I should add most newer vehicles wont engage to OD unless it's safe and beneficial to the driving situation on most OBD2 cars just leave it alone uless towing the TCM will keep track of when to engage and not. Always turn off when towing
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