I recently read that it could be harmful to the automatic transmission to coast in neutral down a hill. Is this true?
It could well be harmful to your brakes, and in case of a steep mountain downhill, very harmful you HEALTH.
Cars are made to stay in gear when you slow down or go downhill. The engine braking prolongs the life of your brakes and prevents them from overheating and possibly fading, causing a potentially fatal accident. On a very steep hill, the owner’s manual normally tells you to put the car in a lower gear, or at least take it out of overdrive.
I don’t know why you are asking the question, but, NO, it will not save you any gas by taking the car out of gear. The computer on a modern car normally either cuts off the fuel supply or greatly diminishes it on a downhill run. If you take the car out of gear it may actually USE MORE GAS!!!
Vote two for all of the above Doc.
It’s unsafe, it’s probably illegal, and it probably will use more gas. What’s the advantage in doing it?
besides all the above, shifting from drive to neutral and back to drive adds to the wear and tear of the transmissiion.
Unless you drive a Toyota, you don’t ever really need to put it in neutral, except when you’re switching between reverse and drive
Don’t do it. It accomplishes nothing and it unsafe.
OK, here is a contrary opinion from a (sometime) hypermiler.
- there are often downhill slopes where the vehicle WILL NOT gain speeds that will require heavier use of the brakes.
- even when excess speed develops, one can ALWAYS go back to D.
- there are times when you actually DO NOT WANT the extra engine braking.
- while the EFI will cut off even the gas for idling, this has to be weighed against the LOSS of kinetic energy from UNWANTED engine braking.
I agree with all of your statements except for the unfounded last one. But the first three are exceptions to the premise of “normal” driving situations. Yes, glare ice is better handled by abs and no engine braking. But the OWNER’S manual is the definitive authority and the engineers who design auto trans would shake their heads at the collective bogus ideas people have that actually shorten the life of a transmission; this frequent unnecessary coasting being one.
The harm comes when you shift the car back into gear while it is moving. It won’t happen all at once, but when you put it back into gear, the whole drivetrain takes a jolt, from the torque converter all the way to the CV joints or rear differential, including the transmission in between. These components are built to withstand normal use (like backing out of a parking space), but shifting out of and into gear at highway speeds more than necessary can lead to additional stress on your whole drivetrain, including the transmission.
OP is not engaged in a Mobil Gas Economy run. Our interest is in his overall safety and the also the well being of his car. A very gradual downhill is one reason engine braking to save the brakes may not be needed, since you would also not need the brakes. These occur very infrequently.
AS pointed out, constant shifting back and forth will wear out the gear shift mechanism and is also very distracting to the driver, potentially causing accidents. Agree that on glaring ice, the ABS and traction control (if so equipped) MIGHT be more beneficial, and engine braking is not needed as much
On item (4) you are wildly speculating; the engine braking is wanted nearly 100% of the time, and loss of kinetic energy is a the result of slowing down (Physics 101), i.e. engine braking is DESIRABLE!!!
This thread is not about hypermiling, a very dangerous activity in the hands of a non-professional. It is about safety and what’s best for the car’s longevity, and not being a nuisance to other drivers on the road!!.
Contrary opinions are valued, thanks for posting.
there are often downhill slopes where the vehicle WILL NOT gain speeds that will require heavier use of the brakes. …I know the point to coasting is not to pick up speed but any gain will not be measurable. Any gain would be consumed by going back up the hill since one cannot go downhill only.
even when excess speed develops, one can ALWAYS go back to D. …no one plans to get into trouble or have an accident but when you need to drop it back into D for some emergency acceleration then thats when something will happen, play it safe and drive in drive
there are times when you actually DO NOT WANT the extra engine braking. …then depress the accelerator slightly.
while the EFI will cut off even the gas for idling, this has to be weighed against the LOSS of kinetic energy from UNWANTED engine braking…not enough to measure or enough to worry about.
But those are just my thoughts from a non-hyper miler/ non Richard Petty type driver.
“3) there are times when you actually DO NOT WANT the extra engine braking. …then depress the accelerator slightly.”
Though I endorse your overall thoughts wholeheartedly;
this really is the only very rare occasion when putting trans, manual or auto, into neutral is advantageous. Been there on glare ice, hard pack or wet snow that fully clogs tread going down grade with abs and trac control. It does work better to have in neutral and fully use the abs only. Engine braking cannot be modulated to retain steering control as well we freewheel alternating with the brake pulsations of abs in these extreme conditions. Just practical physics, plane and simple.
Speaking about “hypermilers” and how fruitless it is. From Prof. Dick Hill, local energy guru in this area on PBS(I can’t take credit). If you’re trying to make a statement and save our oil reserves and not as a personal savings measure, does he have news for you.
The amount of calculable reserves we have is based upon the cost of extracting it from the ground. So as the price goes up, the more reserves we have because it’s now worth it to say, extract from shale or side ways drill. etc. The same goes for natural gas.
I never thought of it that way…
Just wanted to throw that in because I have way too much free time.
By the way…Subaru is saying that their CVT is maintenance free…and last much longer than the rest of the car and is “ideal” for hyper mileing (maybe)