Recently, I've developed the habit of using the neutral gear in my automatic Honda Fit when going down slight inclines where I would otherwise have to use the accelerator to maintain speed. I understand that shifting into neutral while moving has no detrimental effects on the transmission and engine directly, however, I also shift back into drive sometimes while the car is still moving - I try to match RPMs as best I can guess before moving it back into drive. The questions I have are: Is shifting back into drive while moving harmful to the engine or transmission? And if so, can RPM matching mitigate effects? Or should I simply stop doing this all together?
This is a bad idea for several reasons. First, you run the risk of a runaway car. Second, you risk damaging your transmission, which could cause the runaway car in the first place. Thirdly, in newer fuel injected cars, you can actually get worse gas mileage. Let me explain.
As stated before, by not having it in gear, you run the risk of your car getting away from you. You might have to entirely rely on your brakes to slow your vehicle down. While this may not seem like a very likely thing, using your brakes too much down hill will cause extreme heat to build which could cause a reduction in brake effectiveness, which might not allow you to slow your vehicle after a point. Using your engine/transmission as an engine brake will alleviate some of the creation of some of this heat, which will allow your brakes to work more efficiently when needed.
After running downhill without your transmission in gear, you put it back into gear. By dropping it into gear at speed, you are putting a lot of stress onto your transmission as it is forced to bring the torque converter and engine up to speed quickly. Transmissions are only made to withstand so much torque before they fail. Even if you don't exceed this theoretical maximum, you are still dropping a great deal of torque back through the transmission. This has an accumulating effect. Noted race car driver/designer, Carrol Smith, wrote a book called Engineer to Win. Chapter seven is entitled, "Metal Fatigue - or Why Things Break". It reads:
"... under repeated (cyclic as opposed to continuous) stress, the capacity of a metal to withstand stress gradually diminishes and, in most cases, cannot be restored. Metals which are subjected to fluctuating loads can and do break after a finite number of load cycles (or, more accurately, stress cycles) in which the loads applied and the resultant stresses imposed are always below the ultimate strength of the metal. This type of failure is termed 'fatigue failure'."He states at lower stress levels, things can be made to last indefinitely, but when you start putting those higher loads on the parts, they will break sooner. It is not linear in nature, either, the higher loads you force upon it, the sooner it will happen. By dropping it back into gear from neutral, you are putting these "higher stress levels" through the hard parts of your transmission, introducing the stress that would not otherwise be there.
Within the computer of fuel injected cars, there is a table which is read when the throttle position sensor reads that the throttle is closed (or that the gas pedal is not depressed) and yet the engine is still going at an RPM above idle (this is a simplistic way of looking at it, but basically the way it is). When it reaches this predetermined state, it will drop the injector open time to zero, meaning that no gas will be flowing into the engine. While this period of time is probably not very long, your engine will still use more gas by idling while you have it in neutral than having it in gear and allowing it to attain this state.
From my experience, it depends on a vehicle and driving conditions: - Toyota Corolla, driving for 8 years and never had a problem with tranny when shifting D-N-D often. Need to be careful when RPM doesn't match speed: for example N-D shift at 0 with high RPM (when need to move, switch to Drive is forgotten and gas is pressed), it may jolt or even break, or when speed is high and engines RPM is below 1000, idle, neutral - this can be easily coordinated by adding RPM above 2k and then switching to D. - Honda Pilot, I never do it because a. steering wheel switch is inconvenient b. Honda tranny's are not very good, fail a lot per stats, and jolt sometimes even from normal use. - Driving in mountains: neutral is never recommended. Even more, stay at lower gear or lowest gear when going long downhill - Driving in winter conditions: could be good idea to stay on D as much as possible, however Canadian winter driving instructors officially recommend switching to N and breaking when emergency breaking is needed - they explain it, and I tested it, by shorter breaking distance when engine is not engaged.
By the way gas saving on N are not significant, zero when N on stop lights, and a little bit like 1-2 miles per gallon when going non-steep downhills.
I've done that a number of times while trying to diagnose odd noises that only occur at speed. Nothing bad has ever happened, but those times are all spread across a number of cars, so the times I've done it per car is very low. Could have a cumulative effect? Like @Nate says, it's technically illegal in many places. I can't recommend it as standard practice either way because it does put the car into an unusual state and could make emergency handling more difficult.
Ahh - the wonderful benefits of owning a manual. One can disengage the engine at any time without worrying about a complicated automatic transmissions. I, for one, think it is a complete waste of gas when one has to press the gas on an automatic to go down a long hill.
Knowing nothing about automatic transmissions, one has to take Paulster at his word about the potential damage to a transmission shifting in and out of drive while moving at significant speeds. However, I have done it for years on a variety of vehicles (mostly GM and Volvo), and never had a transmissions problem. And, I typically drive all my cars to 175,000 miles.
The only point that Paulster is really out to lunch on is the point about the car getting away from you as you go down a long hill. If one is aware enough to shift one's car out of gear, one is also aware enough of gaining speed, and the ability to shift back into gear (and downshift in either the automatic or the manual.) If you are going so fast that it becomes dangerous, you simply should not be a licensed driver.
I don't think it is a good idea. Besides what's being mentioned, say you want to suddenly speed up to avoid an accident or an unpleasant situation, with N in use will def make it difficult to respond quickly. I used to do it myself, but I stopped after thinking of it. Hope you find this beneficial.
I got this reply from TopGear...
Quite a number of drivers, particularly the ones who picked up bad driving habits from other drivers who didn't know any better, often practice improper driving by not shifting to neutral when the car is not moving. While I would like to tell you that there is a good reason (under normal circumstances) for keeping it in D and stepping on the brakes at a stop, I can't. It's mostly born out of laziness and improper training.
As you have correctly surmised, the engine does consume more fuel and you do wear down your brake-system components more when you leave your transmission in D or Drive while the traffic light is red or when you’re idling and waiting.
With automatic transmissions, the engine is energizing or driving the transmission to move in the direction of the gear you've selected--forward or reverse. When you keep it in gear, you are telling your vehicle to move; when you're on the brakes, you're preventing motion. You're unnecessarily and slowly raising your automatic transmission fluid, wearing out your transmission clutches, and consuming more fuel because you're on the brakes to keep the car from inching forward. You're also unnecessarily wearing down and heating up your brake pads. By simply shifting to N or neutral there would be no need to apply the brakes with the amount of force required to resist forward motion.
If you notice all of the above result in unnecessary wear, all of which increase the cost of operating and maintaining your vehicle. It's also a very unsafe practice as if you happen to accidentally lift your foot off the brake, you will most likely get into an accident.
While we're on the subject of automatic transmissions, it is also a very bad habit to shift into P or Park when you're at a stoplight. Numerous drivers have adopted the practice as well and, quite frankly, it is also an accident waiting to happen apart from slowing damaging a different part of your transmission, which will eventually lead to the a different sort of automatic transmission failure.
It won't matter if your car has CVT or the conventional multi-speed automatic transmission. Shifting to neutral under most normal conditions is good practice.