Is there any benefit to turning off my car's overdrive?

April 11, 2007 5:29 AM

I recently bought a car with a button that allows me to turn the transmission's overdrive on & off. I've read that overdrive only kicks in to provides fuel economy when travelling at highway speeds. Is there ever a reason to turn it off?

The car is used for my commute on city surface streets - I hardly ever get it over 50mph.

posted by Tasanova to travel & transportation (15 answers total)

There were situations, when I was driving my 02 Mustang, that the computer would get confused when I was driving around 45/50 - it would drop into overdrive, lose torque on a slight uphill, and kick back to 4th...for about 5 seconds, and then back to 5th again.

This was about a once a month occurance, on a specific road. Did it hurt the engine? No. Did it annoy the ever-loving crap out of me? Sure did. So, the option was there, and I used the button.

This, as far as I know, is the most common usage of the feature. However, it is also provided for situations where you need more torque - overdrive runs the transmission gears to the point where the wheels are turning at a higher RPM than the engine, which is where your fuel economy boost comes from. However, the power is very diminished; while it doesn't take much to keep the car moving, if you're towing something (which, even on a Mustang, is technically possible) you're going to need that torque. The computer isn't smart enough to figure out that you've got extra weight, and won't react properly in some situations.
posted by plaidrabbit at 5:55 AM on April 11, 2007

Yeah. I have a Pontiac Vibe with that arrangement and for city driving I keep it off because I get faster acceleration with the button off. For a 4 cylinder car, I need all the acceleration I can get. Overdrive helps even at lower speeds (40-50mph) when you want to cruise without much engine braking but for normal city driving, leave it off.
posted by JJ86 at 5:59 AM on April 11, 2007

No, no reason to turn it off... unless you have a "gear hunting" problem like plaidrabbit describes.
posted by Doohickie at 6:12 AM on April 11, 2007

I have a Pontiac Vibe with that arrangement and for city driving I keep it off because I get faster acceleration with the button off.

You shouldn't, unless hitting the overdrive button on a Vibe also changes the programming of the engine. Your gear ratio in first, second, and third should be the same whether you've got the top gear turned on or off.

Hunting or towing are two reasons to turn it off. Another is if you're on a long, slight downgrade and you want just a smidge of engine braking.
posted by ROU_Xenophobe at 6:16 AM on April 11, 2007 [1 favorite]

Turning off the overdrive is one way to get a little bit of engine braking, say on long highway descents or something. Last I knew, the jury was still out on whether engine braking was even a good idea, though, so this may not be a desirable use of the feature.
posted by box at 6:16 AM on April 11, 2007

In situations where you want "predictability" -- such as adverse weather -- it's good to turn off overdrive so that the (minor) torque jolts that come from the overdrive engaging/disengaging don't cause a loss of traction.
posted by u2604ab at 6:52 AM on April 11, 2007

I turn overdrive off if I am driving in a very hilly area. Otherwise there is a shifting of gears as the car gains or loses speed. By turning it off going up or down steep hills keeps the shifting to a minimum. And as someone said you get better engine breaking with it off.
posted by JayRwv at 7:00 AM on April 11, 2007

Turn it off when you want to go fast or will need extra power for hill climbing or the like, otherwise leave it on for economy.
posted by caddis at 7:10 AM on April 11, 2007

in all of the cars i have owned that had auto overdrive, they recommended not using it in slippery or snowy conditions.
posted by lester at 7:42 AM on April 11, 2007

Last I knew, the jury was still out on whether engine braking was even a good idea

In city driving,not on downslopes.
posted by ROU_Xenophobe at 7:58 AM on April 11, 2007

Is it a Saturn? My Saturn has a button like that and I don't think it does a damn thing.
posted by scratch at 8:07 AM on April 11, 2007

Engine braking is a joke with an automatic tranny. With a stick though you can finely tune your speed in traffic, rarely having to touch the brake pedal. Going down hills many experts seem to prefer regular braking, but I get the impression that the extra stress on the engine is still pretty minimal.
posted by caddis at 8:08 AM on April 11, 2007

So automatic transmissions have a do-hickey called a torque converter. It transfers the torque of the engine to the transmission via a fluid medium (transmission fluid). Picture stirring cake batter w/o holding the bowl. Your stirring is the engine and the spinning bowl is the torque that goes to the transmission then wheels. The more load there is on this fluid coupling (lots of stirring, not much bowl spinning) the more the fluid heats up because of all of that friction in the fluid. The fluid (and thus your transmission) breaks down faster if that fluid gets too hot. When your driving normally in OD, cruising down the hwy there isn’t much or any load (some transmissions “lock out” the torque converter on the hwy) on the torque converter (the bowl turns at about the same rate you stir). If there is a load however (going up a long gradual hill, towing a trailer, anything that is slight enough to not make the transmission downshift to the next lower gear), then you start heating up the transmission. This is fine for a few seconds/minutes, but if you’re towing a trailer, or driving over big mountain passes with your car loaded with all your crap from school, it can heat up your transmission. When you turn off the OD it lessens the load on the torque converter by not allowing the transmission to use the highest gear. This makes your transmission cooler and happier, it also, however, decreases your fuel economy.
It’s also handy for engine breaking so you don’t have to use the breaks so much and have a bit more control of your speed going down a long hill. But, really the only time you ever have to worry about “saving” your breaks is if you’ve got a big load. Basically don’t worry about it unless you’re towing the boat.
It might give you a little more immediate acceleration because when you step on it (say to pass) it saves the time it takes the transmission to kick down to the next lower gear, but driving around with the OD off ‘cus you like that punchy feel, wastes a lot of gas.
Too much time on my hands today…
posted by sauris at 8:19 AM on April 11, 2007

I seem to recall that, when we drove up the side of Mt. Washington a few years ago, the rangers advised us to turn overdrive off. I wouldn't swear to it, though.
posted by fogster at 8:51 AM on April 11, 2007

Thanks for taking the time sauris... or giving it, rather.
posted by Tasanova at 10:46 AM on April 11, 2007