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O/D is exactly opposite of what you are saying. O/D will not downshift to the lower gear. In O/D you shift to the higher gear thus helping engine run at a lower RPM and better FE. Technically, O/D means the engine speed is lower than the transmission speed i.e., the gear ratio is less than 1. It will not help you overtake a truck faster, since the torque available will be lesser.

I am posting a link to this article which explains O/D.
What is the transmission’s overdrive? - Ask.cars.com

+1 to that. I see too many people keep the car on "D" mode when waiting in a traffic light with the brake pedal pressed. I am sure it puts too much pressure on the brakes. No wonder brake disks and pads wear out so fast in automatics. I am sure it wastes fuel too. I am new to driving automatic, but I figured out I should be in "N" and apply hand brake. Needs some getting used to, but trying to make it as a habit.

Putting the car to N or D (with the brake pressed) doesn't make any difference to the fuel wasted as engine will be just idling at constant rpm. It is only when you take of the feet from the brake the revs rise.

Can you please support your claim with a proper reference ? You are making a big statement and if it is true, it is important for people to know and follow.

I don't think brake should have anything do to with engine RPM. But with these advanced technologies it is hard to assume anything. I will check on this tomorrow and report. Really simple to check. Keep the car on D+Brake and N and watch the RPM meter.

Guna: Checked it. Putting the car on "N" or "D" without accelerator input is just the same. 750 RPM in my case. Only difference is in the "D" the car moves and in the "N" it stands still. I can't say whether the fuel consumption is same or not in these two cases through. My car does not have an instant fuel efficiency meter. Again, I don't want to assume same RPM means same fuel consumption. If someone with an instant fuel efficiency meter can report (hoping it is accurate), it will be great.

This is an awesome thread!

When test driving the Vento AT recently, a couple of us (independently) experienced a jarring noise when the brake lever is released after putting the car into D. The noise happens at the point of releasing the brake when it is fully depressed If you don't FULLY depress the brake while changing the gear, it doesn't give this noise.

Is this normal? I don't remember this being an issue when I've driven the OHC AT years ago on any of the times I've driven automatics in the US. Stalwarts please advise.

my car has the instant meter but it remains blank when stationary (since there is no distance covered, KMPL would be zero :)). True, same RPM need not mean same fuel burnt as the computer can (if designed to ) feed more fuel to maintain the same RPM.

Here are the exact words in my Civic-AT manual:

"Use N if you need to restart a stalled engine, or if it is necessary to stop briefly with the engine idling".

So it is okay if we put in in 'N' for a long wait at signals (with the handbrake on for safety).

that sums it up perfectly!
N for traffic light situations(long and short) - D and brakes just lead to wear and tear.

In terms of consumption of fuel - there is no difference...

While stopping at a signal, its better to put in N + Hand Brake rather than in D + Brake. It will help to avoid sudden moves forward and protect us from road rage.

How about having an Idiot's guide to Automatics? With the number of auto tranny user on the rise everyday, I am sure it would be a big boon, especially to those who are first time drivers. It would be great if the Mods could come up with a detailed write up on the above topic.

+1 to that. After we got got an automatic in the house, its been a learning curve on how to use the automatic in the most effecient & correct manner.
i.e. Anyone can drive an automatic, but how to drive it correctly has been a little cumbersome by asking & reading up on it on various sources including the service advisors when the car goes for service.

Generally people think theres nothing to it but dont realise that it could cause harm to the car & that they are getting lower FE due to the incorrect usage rather than because its an automatic.

Case in point: Most people keep the gear in "D" even when halted at traffic lights while just using the brakes. The correct usage is to put it into "N" and let go of the brakes. This incorrect usage results in increased brake wear & higher fuel consumption.

P.S. - A query here is that ive felt that in automatics, the car doesnt decelerate as fast as manuals on letting go of the accelerator. Is this just me or is it technically right?

:OT but can you explain why you feel halting at traffic lights with the gear in D would lead to increased brake wear and higher fuel consumption! Putting an AT vehicle in N and letting go of the brakes is the same as putting any geared vehicle in Neutral and not braking. This is DANGEROUS.

And btw, if you want to really "save fuel and reduce brake wear", put the vehicle in P and switch off the engine.

Thought this might crop up.

Well, what i meant was that on a level road we should put it into N without the brakes (for a short signal, etc.)
But even on a non-level/incline, it can easily be put into N (keeping the brakes pressed just like you did in D or have the handbrake engaged just like in a MT).

Generally, people just leave it in D & ride the brakes for short stops to avoid the small procedure of moving the gear selector & pressing the brake to do so.
But you should actually select N for short stops while keeping the brake pressed if you are on an incline. If you are stuck in dense traffic or are at a long traffic light on an incline, then you can obviously put it into P.

WHY ITS BAD FOR THE CAR:
The reason for D leading to increased brake wear & higher fuel consumption is that if you notice, when in D, even when the accelerator is not pressed, the car moves forward. This is akin to using just the clutch to move forward in a manual in dense traffic conditions. Now as we all know that this leads to higher fuel consumption but since the brakes are keeping the car in position while the engine is trying to push it forward, this riding of the brakes leads to increased brake wear & heat build up. Another point i forgot is that this also strains the engine.
While keeping it in N with the brakes pressed, the engine is not trying to move the car forward, hence no fuel consumption or break wear or engine strain.

In a nut shell, keeping an AT in D with the brake pressed is like half-clutching while keeping the brake pressed in a MT. Would you ever do that in a MT?

WHY ITS A SAFETY HAZARD TO KEEP IT IN D:
Coming to the safety part, having it in D with the brake is the actual safety hazard as if the brake pedal is left unintentionally, the car will move forward. Hence, actually, this is the DANGEROUS usage rather than putting it into N (on a level road) with the handbrake engaged/P (on a non-level road) wherein the car will not move forward.

@Neilguy - I did not mention this to save fuel & reduce brake wear but rather to use the AT as its supposed to be used. Correct usage doesnt "reduce" fuel consumption or brake wear but actually keeps it at the level it was intended. Not following correct usage is what "increases" the fuel consumption & brake wear above the actual level intended not to mention engine strain. Hence, relative to the wrong usage, the correct usage "reduces" fuel consumption & brake wear while also keeping the engine strain free & healthy when compared to the wrong usage.

P.S. - This is exactly what i meant when i said that most people do not know the correct usage of ATs (including me when i was a newbie to ATs). No offence meant to you Neilguy.

Sorry, I do not agree. What you do may improve FE and reduce brake wear. But doesn't mean waiting in D is incorrect. I have been using Auto almost continuous for 18 years, in fact I was properly trained by highly knowledgeable instructors before I got my US license back then. But I didn't hear about this switching to N at signals until few months back, probably in this very same thread.

This waiting in N was probably invented by some FE and brake wear conscious drivers. I have nothing against it, but as far as I know I never seen anybody doing it until I heard it in this thread. I am usually open to new things, so I tried it a few times. Every time I got honked by people behind. With D, I just have to lift my foot. With manual shift, I can shift to 1st gear quickly without looking. But in auto, shifting from N to D has to be done carefully, else it will move further down.

Stopping in N in signals is a habit practiced by some people, it has its merit if you are comfortable with it. But, please do not claim that waiting in D is incorrect. I have never heard it mentioned when I drove in US for a decade, where 99% of the cars are auto. I didn't get a point deduction for not putting it in N in signals during my driver's license road test. I have been to many DL road tests in US for friends and family, never seen anybody lose a point for that. Obviously it is not incorrect.

If FE and wear/tear is the yard stick for deciding right and wrong, then anything other than very sedate driving will be incorrect. All spirited driving will be incorrect since it has adverse effect on FE and brake lining.