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I've been told that when needing to brake quickly, throwing the gear into neutral on an automatic transmission actually allows for better/quicker braking. I understand how things work pretty well but lack automotive knowledge -- so I cannot really say if this is correct or otherwise.

If it doesn't help braking, are there any other actions happening that are positive/negative when this is done?

Thanks!

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4 Answers

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Think of it this way.

If you need to brake so incredibly hard that you're worried the automatic transmission is getting in the way, you're better off worrying about things besides whether or not you're in neutral. You're probably about to crash or lose control, so train yourself to concentrate on steering, or making sure you're arms are out of the way of the air bag, etc. Worry about anything but the transmission.

The amount of power being delivered to the wheels by an automatic transmission is so tiny and insignificant when your foot is off the gas that you should never have to pay it any mind. In theory it might make a difference, but not in real life.

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I totally agree that addressing the gear in a situation where a crash might be avoided due to concentration on steering is what you want to do. I'm just more curious to is happening during this process and if there is any overall net effect in taking the action of changing the gear into neutral. – shaselton Jan 29 at 19:43
There's not any magic going on behind the curtain. You are either in neutral or you are in gear. Automatic transmissions use a torque converter allow you to come to a stop in gear, so the forces of your idling engine are negligible. If you want a lot of detail you should read up on torque converters. – hillsons Jan 31 at 5:50
As an alternate view on use of the transmission in stopping situations, I once avoided a bad rear-end collision scenario via shifting an automatic. I didn't realize it, but my left tires were right on a metal joint in heavy rain, and the ABS system seemed to detect that as slipping and refused to brake hard. So I threw the transmission in park and stopped almost instantly, with a horrible noise from the transmission and (possibly unrelated, I dunno) busted transmission seals and lost gears a year or so later. – R.. Jan 31 at 6:03
good answer. Whenever these "when braking for an emergency, should I..." questions come up, the safe answer is always: Use. The. Brakes. To. Stop. The. Car. Also, know ahead of time whether or not your car has ABS. If it does, simply mash the pedal as hard as you can, then push it a little harder. Find the safest exit and point the car there. – mac Feb 1 at 15:34
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This sounds like utter nonsense to me. Try this experiment. Get up to a set speed (e.g. 60 mph) and then let your foot off the gas pedal and time how long it takes you to get down to a low speed (e.g. 20 mph) "coasting" in gear. Now, repeat the experiment putting the automatic transmission in neutral at the same time you take your foot off the gas, so that you're really coasting, and see how long it takes. I'm betting it's a lot longer.

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You bring up a good point. The forces in the engine might even slow your car down faster than simply coasting in neutral! – hillsons Jan 31 at 5:44
this will depend on the transmission and whether or not it is in "lockup" mode when coasting. Also, just because it's in lockup while coasting doesn't mean it's in lockup while braking (or vice versa). – mac Feb 1 at 15:29
Even in non-lockup, the torque converter will slow the vehicle down. – R.. Feb 1 at 15:39
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Putting it in neutral will somewhat reduce your available braking force, as you're removing engine braking from things.

No power is going to go from the "idling" engine to the wheels in anything relatively modern. Basically every fuel injected engine since the 80s implements deceleration fuel cutoff (DFCO), which stops injecting fuel if the wheels are in motion and your foot is off the gas. As a result, the engine is only kept spinning by pulling energy in backwards, somewhat braking the vehicle.

The benefit is that by removing the engine from the equation, you get much more controllable and predictable braking, which is important in extreme conditions, like stopping on ice, especially if one doesn't have anti-lock brakes.

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Engine braking on an automatic will only occur if it has locked the torque converter. Otherwise the engine can only help the car accelerate, but the drivetrain is "free" so to speak. – Nick Jan 31 at 14:49
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If you can hit the breaks hard and quick enough to lock the wheels or activate the ABS with no discernible delay then the engine and transmission is no issue.

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