overdrive in cars/automatic transmission

Factual Questions
1

Oboy. This is an embarassing question:
All my adult life, I’ve driven a car with manual tranmission. Just bought an automatic.
First: there’s a button I can press on the stick that’ll shift me into overdrive. What does this do exactly? What are the advantages/disadvantages of overdrive? Power vs. Mileage?
Second: adjacent to the “Drive” setting, there’s the numbers “1” and “2”…when would I need to use these? Is this first and second gear? Why would I need these, unless…
Third:…there’s a way to push/roll start my car? If I wanted to and if this is what these settings are for, how do I accomplish this? Start in neutral, then suddenly throw it into first?

I feel like such an idiot for not knowing all of this, but quite honestly, I’ve never needed to know this until now (sorta like the time a district manager came into my store and asked me to make coffee for him. I told him that I didn’t know how. He freaked out and went into hysterics about it. Not being a coffee drinker myself, it was just never ever necessary for me to know this procedure…I can’t make a martini either).

A little help, please…?

2

For nearly all driving situations all you will need is drive. The overdrive is for long stretches on a flat highway and first and second gear are for when you want to override the automatic features of your automatic transmission for more power.

All this information should be in your owners manual. If you did not get one with the car you can get one from any dealer.

3

The overdrive basically is a top gear, aimed at low revs, and consequently better mileage. It’s useless when accelerating or climbing, since the engine is NOT at its peak torque at these lower revs. Also, an overdrive only really works with a sufficiently powerful engine. Not that the average American car lacks power :wink:

You can’t push-start an automatic, as far as I know.

4

I was told once you could push start an automatic, but it had to be rolling at something like 25 mph. Considering you’d have no power steering or brakes, rolling at that speed would be dangerous, even if you could sustain it long enough.

Unlike a manual transmission, the gears are there to limit what the car can do. In an automatic transmission, first gear, second gear and drive all mean your transmission won’t shift PAST those gears, not that you can start out in them (as with a manual). That’s effective if you’re rolling down a long hill, or on slick pavement when too much power might make the wheels slip.

You lock out overdrive to make sure your transmission doesn’t have to shift back to the power gear. The typical example is when you’re in heavy traffic, or on a hilly road, between about 35-45 mph. The transmission wants to shift into the higher gear, but every time you step on the pedal, it downshifts back into drive. Locking out overdrive keeps the car in drive and eliminates the downshifting.

5

I’ve always driven manual transmissions, too, but it is my understanding that:
First: what Coldfire said.
Second: your automatic tranny probably has 3 gears plus overdrive. If you have it in 1, then the highest gear that it will go to is 1st. If you have it in 2, the highest gear that it will go to is 2nd. If you have it in Drive, it will go through all 3 gears as needed. You would normally only use 1 or 2 if you are hauling a trailer or something and needed the engine to rev higher to get moving, or if you were going down a steep hill and wanted to use the engine to brake with.
**Third:**what Coldfire said.

6

I don’t think it was made very clear that 1st and 2nd gears are basically for very slick conditions in which you need extra torque, not for going slow. You can but on some models it will ruin the transmission.

7

In defense of poopah, I’ve read our CR-Vs manual, and it doesn’t explain Overdive for crap. I was reding it because my husband was telling me that OD was how to engage 4-wheel drive in our CR-V, and I think he’s wrong.

However, it is a good idea to read that thing cover to cover. It’s amazing what you might never know about your car.

You’ll probably rarely use those other gears. I shift down (to 1 or 2) when I’m going down a steep hill and find myself riding the brake, or when it’s unsually slippery. I never tow anything, but that would be another time to use it. Your owner’s manual will tell you what speeds you need to observe while in those lower gears.

8

3-speed plus overdrive? Amazing, the advances they’ve made…(I driva a '71 Nova w/2-speed Powerglide auto :D)

9

Here’s a few details that haven’t already been covered. The overdrive is a final-stage overrunning device at the tail end of your transmission. It’s a tricky set of planetary gears which makes the driveshaft run faster than the output shaft of the transmission. The engine runs slower in OD, so you save gas. However…when you take your foot off the gas, the OD will freewheel instead of letting the engine slow you down. On a long downhill, that’s dangerous. All your slowing will be handled by the brakes, which will heat up and become useless after a while (the brake fluid boils.) Another however…if you’re hauling a big trailer, the OD might keep the engine at a gas-saving speed when you really need the power of a higher engine speed to haul that big cabin-cruiser. That’s not good for the engine.
MOST of the time, you can keep the shifter in top gear with the OD engaged, and the machine will make all the right decisions for you. You may find that boring, but if you try to play Stirling Moss by blipping up and down through the gears, you’ll waste gas, and you might risk hurting the automatic clutches inside the transmission.

10

I used to work in Automatic Transmission design at one of the Big 3. Here’s what I know.

First, What AskNott said…and…

OD–saves gas, and good for flat driving where wheel torque requirments are low. This means that the vehicle will shift 1-2-3-4 (in a 4-speed model) and will continue to upshift and downshift as dictated by the shift schedule.

OD-off…Transmission will shift to 3rd gear and will not go past 3rd gear. Provides engine braking for downhills and more torque at wheels for uphill driving.

Manual 2. In the transmissions I worked on, Manual 2 means, the vehicle will operate in 2nd ALL the time. Even if you stop and start again. If you are moving when you engage manual 2. The vehicle will shift into second at a safe vehicle speed and engine speed.

Manual 1. The transmission will stay and start in 1st. Same thing as 2…just stays in 1st. Again, the vehicle will wait for a safe downshift speed if the vehicle is moving.

And I have never heard of a way to push start an automatic…being that I think you would need fluid pressure to get the wheels to turn the engine. But I could be uninformed on this.

11

AskNott and Jeel, among others, are right. The only remaining question is about push-starting an automatic. That cannot be done at low speeds because there is no direct mechanical connection between the engine and wheels. Power goes through a torque converter that is usually compared to 2 fans facing each other - if you switch one on, the air flow pushing on the other makes it turn, too. In a car, oil is used instead of air, and it recirculates inside the torque converter in a complicated way, but that’s the idea. If you start the drive wheels rolling at any reasonable speed, all you’ll do is spin the wheel-side “fan” without getting enough torque into the engine-side one to turn the engine over. Theoretically, if you get going fast enough, it’s possible, but you still be safe in trying it. With a manual, instead of a torque converter, there’s a clutch consisting of 2 friction disks that mash into each other when engaged. That arrangement allows enough torque to get into the engine to crank it.

12

I should add that recent automatics (in the US, anyway) generally have devices that do mechanically lock the 2 sides of the torque converter, whenever the electronic doohickeys conclude that you’re at a constant highway speed. The lockups eliminate power losses due to torque converter slippage and improve mileage a bit.

13

If you are going down a steep hill, especially if you have a heavily loaded vehicle, you will learn to love the 1 and 2 positions of your automatic tranny. It’s much better to use the tranny on prolonged downhills than to use your brakes, which can overheat and boil your brake fluid. By preventing the tranny from upshifting it keeps the revs high and increases the engine braking.

Also, some luxury cars have a winter driving mode, that starts the vehicle in second, rather than first. As I understand it, this is to limit the torque to the drive wheels when starting out and prevent slipping. I imagine this is more common on powerful rear wheel drive cars.

Finally, unless you are going down long hills where you need some engine braking, or driving around town at just the right speed that your tranny is hunting up and down to overdrive, leave the OD button on all the time. There’s no reason to deactivate it otherwise.

14

Are you saying it will then even accelerate from a full stop in two?

Most of the automatics I’ve driven were either European or Japanese (a few Americans). From what I recall, “Manual 2” stands for “using the lower two gears”, not “using only second gear”.

15

Yup.

16

That’s what the owner’s manual of my late Wagoneer (MoPar 727 3-speed auto) said. I dunno about Ford trannies, though…

17

Could that be an American thing then?

18

It might help if you told us what you drive, since different manufacturers have their own favorite ways of doing things. Also, there are sometimes different transmissions available in the same model, so one might have a 3-speed with (or without) an overdrive gear, and another might be a 4-speed with 4th having an overdriving ratio.

I drive a 1994 Dodge Caravan, and I’m still trying to figure out just how it’s supposed to work (“put it in drive and just go” works just fine 99% of the time). I have a column-mounted gear selector, and can choose from P-R-N-D-2-1, plus I have an overdrive lockout button (ODLB) located on the dash. If the ODLB is not activated, the transmission will start in first, then shift through second, then third, then (finally) to overdrive. In my vehicle, I suspect this is actualy an extra set of gears at the output shaft of the transmission (as described above). Some cars just have an extra gearset or so to give a fourth or even a fifth ratio, and gear them to drive the output shaft[s] faster than the input from the engine.

If the selector is in “2”, it starts in second, but will shift to third (possibly the overdrive – I don’t lock it out most times). When I select “1”, it starts in first and shifts to second (once again, maybe the overdrive). My van isn’t quite like new and I’m not certain if it’s really supposed to work this way, and since the owner’s manual is kind of vague about how it operates, I can’t be sure. From perusing the manual (it’s different in many cars) I do find that they recommend I leave the ODLB alone unless I’m driving in hilly/mountainous terrain or traffic where the transmission is frequently shifting in and out of top gear. Locking out the overdrive will keep it from shifting into overdrive.

As far as driving on slippery surfaces is concerned, what you are ideally trying to do by “downshifting” isn’t to put more torque to the road, but less. Starting in second, rather than first, puts less power to the slippery surface, reducing the chance that one tire or the other will slip. Unless you have a limited slip differential, a slipping tire means all the power from your engine is wasted turning he wheel that’s slipping – no useful power goes to the tire that still has traction. Reducing the power going to the wheels by starting in a higher gear reduces the chance of one wheel slipping. This allows you to start moving sooner with less drama than in a lower gear. When starting from a stop on a slippery road surface, you don’t want to start in first, but second. Leaving the car in “D”, you’ll start in first gear (as will shifting it to first).

Some of you manual-only people might protest that starting out in a taller gear than normal will cause the engine to “lug”. Don’t worry about it. A car with an automatic transmission can’t lug its engine. Otherwise, you’d have to shift into neutral whenever you came to a stop to keep the engine from dying (I’ve owned cars that worked like that, but it wasn’t the transmission’s fault).

~~Baloo

19

FYI…

You should see this listed on the original window sticker of the car – it will be listed as a “lockup torque converter.” This is generally only used on cheap cars with 3-speed transmissions. I guess they figure people who buy cheap cars need to save money on gas. Anyway, it still serves as a type of quasi-overdrive. I think this is how my Ranger works (I’ll investigate); I do have an “over drive off” option.

“Better” American cars seem to have 4-speed transmissions, with no silly nonsense about overrdrive or lockup torque converters. My Bonnie has four real gears that you can only barely feel shifting. Not having a lockup torque converter or a “true” overdrive is a blessing, in that there is no extra hesistation when you need the power – you step on the petal and it instantly goes to the gear it needs to. There’s not winter driving mode; I have some type of traction control that seems to be controlled by the transmission (my old '94 Bonnie used ABS system for traction control, and didn’t work too well).

From my experience, I would recommend that you always look for a 4-speed withOUT the overdrive or the lockup torque converter. Obviosly, that goes only if your needs are similar to mine – I don’t pull anything, and I like to get enjoyment out of driving. The Ranger I mentioned above is NOT fun to drive, and I think it’s the transmission’s fault (hey, it’s a commuter car).

Tschuess.

20

A quick question to all the car buffs: can you shift from drive into overdrive/1/2 while going at any speed? Having driven an automatic for quite a while, I only put my hand on the gears when I start or park my car. I imagine if I find myself running down a steep hill one day I might want to shift to 1/2, but then do I have to come to a full stop first?

Thanks!