Neutral while driving?
February 28, 2007 6:09 AM   Subscribe

Is it OK to switch to neutral while driving a car with automatic transmission?

I've seen the posts where this question is asked regarding "coasting" for manual transmission. But how safe is it for auto transmission if I wanted, while in "D" position, to just bring the car to neutral when I'm downhill or at a red light?

And does it really save gas for auto transmission?

I have a Toyota Corolla 2005.
posted by jadanzzy to Travel & Transportation (34 answers total) 1 user marked this as a favorite
Back when I drove an automatic, I would always drop into neutral when going down hills. I never did any serious measurements to see if I saved gas.

I DID do the measurements on my current manual trans, and I appear to get an extra 20 miles out of a tank of gas.
posted by Cat Pie Hurts at 6:21 AM on February 28, 2007

Oops. To answer your main querstion, it shouldn't be a problem.
posted by Cat Pie Hurts at 6:22 AM on February 28, 2007

I used to drive this Honda that had a really badly placed shifter, so I accidentally hit the shifter a couple of times on the highway and knocked it into Neutral from Drive at 70 MPH. I don't think I saw any adverse results.
posted by jourman2 at 6:29 AM on February 28, 2007

I have done this sometimes when I'm really low on gas and need to squeeze out every ounce...

The only adverse thing that happened was with some vehicles, it's all too easy to keep on going and accidentally shift into reverse. At highway speeds, nothing happens but a really gnarly sound.

I never had any transmission trouble because of it, though.

To answer your question, it shouldn't hurt it as long as you're not on the accelerator while you do it.
posted by chitlin at 6:36 AM on February 28, 2007

It's safe, but there's not much of a benefit to it. Your engine still idles and any wear and tear you think you're saving by not having your engine in gear is made up for by the greater amount of wear switching it back into gear.

No disrespect to Cat Pie Hurts - but from what I've read you will not save gas putting most automatic transmission cars in neutral, since most cars use fuel injection and are set to dispense the same amount of fuel at idle regardless of being in Neutral.
posted by twiggy at 6:38 AM on February 28, 2007

Shifting out of gear shouldn't be a problem. Shifting back into gear while moving might stress the mechanicals a bit, though.

Oh, and Cat Pie...depending on the grade, you should keep your transmission in gear when going down hills. It actually assists in keeping your speed under control, thus saving wear on your brakes.
posted by Thorzdad at 6:40 AM on February 28, 2007

It depends on the car. Subaru specifically warns against the practice, saying it can damage AWD components. the guys at Car Talk say coasting downhill is generally a bad idea. Also: "In fact, it's even illegal in most states to roll down a hill in Neutral with the engine ON!"
posted by Kirth Gerson at 7:07 AM on February 28, 2007

Most modern cars will use less gas when coasting in gear than they will by switching to neutral. In gear, the transmission keeps the engine turning and so no/very little gas is required. Plus, if you get into a situation where you need power NOW, having to fiddle with the transmission isn't something you need added to the mix.
posted by Dipsomaniac at 7:15 AM on February 28, 2007

When your car is in drive, the transmission downshifts as needed so the engine keeps your car from coasting out of control, and reduces wear on your brakes when you're coming to a stop or going downhill.

Plus, shifting in and out shocks not only the transmission, but the u-joints in your driveshaft, or your CV joints, and differentials, and other important bits. Don't do it.
posted by M.C. Lo-Carb! at 7:28 AM on February 28, 2007

Manual and Automatic transmissions are completely different mechanisms. The gearboxes are totally different, require different linkages coming out of the engine, and are controlled differently.

Furthermore, you have a very late model transmission, which is likely to have computer control, variable speeds, etc (I don't know the specifics of the Corolla, but it's a 2005).

Even at a complete stop, shifting in and out of drive stresses the transmission and mechanical parts as MC mentioned. And you waste fuel because engaging and disengaging the transmission requires energy. Transmissions are very very expensive and surprisingly sensitive - it's not worth screwing with them.
posted by Pastabagel at 7:46 AM on February 28, 2007

I seem to remember the Car Talk guys getting a call about sliding into neutral while idling at a light. Obviously, this is a different issue, but their bottom line still applies, I think. The bottom line was that the benefits were few and the negatives were potentially serious: if you're not in gear, you can't get the car to do anything quickly if something should suddenly happen. So, for safety reasons they suggested not coasting or idling in neutral.
posted by soplerfo at 7:46 AM on February 28, 2007

Amount of gas used when coasting in 'D': zero (because the engine computer turns off the injectors completely; the momentum keeps the engine turning.)

Amount of gas used when coasting in 'N': some small amount, equivalent to idling (since the engine would stall out and come to a halt otherwise.)

So, you are in fact wasting gas if you do this.
posted by Rhomboid at 7:57 AM on February 28, 2007

Please correct me if I am wrong (very likely to be wrong here), but I thought that when you are in neutral your drive train is not engaged, meaning that you have less control over the steering if, let's say, you were rear ended? And unlikely scenario of course, but I recall hearing that this is why (in a manual trans) you should downshift instead of slowing/coasting in neutral. And does the same logic apply to auto trans?

Glad you've asked this question jadanzzy! Always wondered.
posted by iamkimiam at 8:02 AM on February 28, 2007

Drivetrain and steering are separate systems, except that the power steering is powered by the engine. If you turn off the engine, the steering gets heavy. Coasting has no effect on the steering.

In case anyone is thinking they are saving wear on the brakes by shifting to Neutral when stopped - they aren't. Brakes wear only when the car is moving. Holding a stopped car does not wear the brakes.
posted by Kirth Gerson at 8:38 AM on February 28, 2007

"And you waste fuel because engaging and disengaging the transmission requires energy."

This makes no sense.

There's a long hill on my way home. Sometimes I put it in neutral and coast on down; I only do this when there's no traffic around - if you're not in gear and need to hit the gas to get out of a situation, that's one more thing you need to do before you can effectively avoid an accident. From a purely technical standpoint, if you've got it in gear and coasting, the engine is running at a higher rate than if you'd had it in neutral. However, whether this means more gas consumption or not is, mostly, moot - two of the inputs that determine fuel delivery are engine speed and vaccum, and those at such low levels, likely balance each other out.

At rest is a different story. The vehicle is not spinning the engine; there's no vacuum increase. In neutral, the engine can produce only so much work as is required to keep itself running. In drive, the engine also needs to spin the torque converter and the transmission pump. *slightly* more work=slightly more fuel use. Another advantage of putting it in neutral at (especially long) stop lights is that you don't have to put your brakes on as hard. This in theory leads to less heat warpage. Anecdotal evidence supports this: My brother-in-law diligently puts it into neutral as he rolls up to any stop. I usually forget to do this. I warp brakes with disturbingly frequency, he doesn't.
posted by notsnot at 8:49 AM on February 28, 2007

Actually, idling, or that is using engine braking, ast higher revs, doesn't use much gas at all. The engine is being made to run by the wheels at that point, rather than burning gas.

It saves more gas to engine brake to a stop than to slip into neutral and use the brakes alone.
That and as pointed out, it's considered highly dangerous to ever be driving without the engine or brakes applied, as it leaves you putting no control in. It will fail you in a test, at least in the UK.
posted by opsin at 8:53 AM on February 28, 2007

Notsnot - I know this is like arguing about religion, but I must respectfully disagree with your assertion that holding the brakes while stopped makes them hot. It doesn't at all. Brakes get hot from the friction between the rotor (or drum) and the pads/shoes that occurs during a stop. Not while the car is stopped.

If you are warping your rotors, you're probably waiting for too long to stop, then slamming on your brakes, which creates a lot of friction. Or your riding your brakes down a hill. This can be fixed by adjusting your braking technique, or purchasing higher-quality rotors, which are available for most vehicles.

Or there is a mechanical problem, such as a bad wheel bearing or a wheel that has been attached too tightly by a careless tire shop employee, thereby slightly warping a good rotor.

You could have your car in drive but stopped all day long with your foot on the brake, and they'll stay nice and cool.

Hopefully your brother-in-law has a special fund set aside for a new transmission from all that extra shifting between drive and neutral!
posted by M.C. Lo-Carb! at 9:05 AM on February 28, 2007

When I was a teenager, I drove the family Chevy full-size wagon for some time. It had a problem where the wipers would turn on, but you couldn't get them to turn off until you actually turned the car off...

So when the wipers refused to go off on the highway, I would lower the car into neutral, turn the key to off (at 60 mph), make sure the wiper control was set to off, start the car up again, and lever back up into drive. Smooth as silk.
posted by joecacti at 9:08 AM on February 28, 2007 [1 favorite]

When your car is in drive, the transmission downshifts as needed so the engine keeps your car from coasting out of control

I have never driven an automatic that did this. All of the automatics I have driven were quite happy to just keep going faster and faster down the hill, even upshifting while doing so.
posted by ROU_Xenophobe at 9:16 AM on February 28, 2007

I asked my father this same question several years ago. He was a trained mechanic in the Army. He said that it's not generally a good idea as it places more strains on the transmission, but I didn't bother to elaborate and get any details. If he calls up today I'll ask him again!

I do notice if I do it in my car that it can jolt when going back into drive if you do it while on the move.. but that's just like a manual and caused by the difference in RPM.
posted by wackybrit at 9:40 AM on February 28, 2007

I don't know if I'd recommend it while driving, but I like to shift into neutral while stopped at lights to reduce strain on the drivetrain and brakes.
posted by tjbarrett at 9:58 AM on February 28, 2007

ok, i just started driving a manual transmission last year and my father tought me despite not having driven a manual transmission himself in 20+ years so i have had some questions about all of this myself in regards to manual transmissions. originally he taught me to downshift while braking as a means of slowing the car down and saving wear on the brakes. but isn't this just putting a lot more wear on the transmission, which is a much more expensive thing to replace? shouldn't i just use my brakes in the gear i am in and then when the car begans to slow down enough switch into neutral and continue breaking till i come to a stop? also, when i am at a light shouldn't i be in neutral and breaking instead of having my foot down on the clutch the whole time to prevent wear on the clutch?
posted by Bengston at 10:36 AM on February 28, 2007

With an automatic shifting into neutral while moving is safe, indeed this is what you should do if your accelerator ever gets stuck. However because modern automatics lack a rear pump extended coasting while in neutral with the engine off will cause damage to the transmission. If theory there might be other internal components that require the transmission to be in gear while the car is moving to be lubricated which would account for Subaru's instruction. So if you were thinking of driving to the top of Pikes peak and then coasting down with your auto in neutral you might want to think again.
posted by Mitheral at 10:59 AM on February 28, 2007

MC lo-carb,,
I know damn well that sitting with the brakes shoved down doesn't create any heat. However, upon coming to a stop from cruising speed, , the pads have a lot of heat. If you are on level ground, with the car in neutral, and no brakes applied, the brake pads either loosely abut the rotors or have a slight gap betweent the pads and the rotors. Convective heat transfer from said pads and rotors will be much less than the conductive heat transfer that would occur if the brakes were firmly applied. Put too much heat into one area of a rotor, and it warps.
posted by notsnot at 11:22 AM on February 28, 2007

but isn't this just putting a lot more wear on the transmission, which is a much more expensive thing to replace?

In theory, yes. In practice, no. Manual trannies are tough little things and can take it, though you can expect it to make your clutch wear marginally faster than it otherwise would. If downshifting normally is enough to fuck up your syncromesh, you are driving a car with one sorry, shitty-ass transmission. I engine-brake extensively in my 1997 Prelude, and it's up to 150,000 miles and doing fine. It still doesn't even need a new clutch. Some of this will depend on who made the transmission, I guess. OTOH, downshifting isn't going to save you very much on brake wear.

Really, why you might downshift is that downshifting keeps your engine close to the power-band so that if Something Bad happens you have a better chance of scooting out of the way. If you're in fifth slowing through 20 and try to accelerate, your engine will be Very Boggy Indeed. In second at 20, you push the go-pedal and shit happens *now*. You will almost certainly never need to do this even once, ever, but then you'll probably also never use your airbags even once.

when i am at a light shouldn't i be in neutral and breaking instead of having my foot down on the clutch the whole time to prevent wear on the clutch?

If the clutch pedal is depressed, the clutch isn't making contact with anything and isn't getting worn. Do you mean the connections between the clutch pedal and the clutch? If you are at all worried about wear to these items, quit driving your car immediately and sell it for scrap so that nobody else is exposed to such a deathtrap.

In theory, you should take the car out of gear and apply the parking brake each and every time you stop your car for any reason at all, in order to limit what your car does if struck by another, but almost nobody does this in the US.
posted by ROU_Xenophobe at 11:33 AM on February 28, 2007

*throwout bearing*
posted by Kirth Gerson at 12:05 PM on February 28, 2007

I do notice if I do it in my car that it can jolt when going back into drive if you do it while on the move.. but that's just like a manual and caused by the difference in RPM.

If you get driveline shock doing this in a manual, you need to work on your technique.

And yes, you can damage your throwout bearing by sitting on the clutch at a stop excessively. Clutch in, put it in neutral, clutch out, foot on the brake or set the parking brake.

(But it's not a big enough deal that I don't do it in rapid stop-and-go, myself.)
posted by jammer at 12:19 PM on February 28, 2007

My dad has this thing with automatic transmissions - he likes to play with them as if they were a stick shift. He likes to shift his truck from Drive to Overdrive while it's moving, and he does the same thing in my mom's SUV, except he changes from low gear to drive. My dad blows transmissions in every single vehicle he drives long-term - in fact, I think his 1994 truck (which he bought brand new in 1994) is on its third transmission.
posted by IndigoRain at 12:58 PM on February 28, 2007

Actually, I learned a long time ago that shifting into neutral on a hill covered in snow/ice is an excellent way to prevent slippage, I do it all the time. The issue is that in drive, there is always forward torque being applied to the tires, and braking at this junction can force a slide. Shifting to neutral removes the forward force (besides inertia) and can help prevent the lockup.

It works a charm, try it.
posted by TomMelee at 1:57 PM on February 28, 2007

Downshifting to save your brakes is a holder from the old days. These days it's much better not to engine brake...and just brake!
posted by toastchee at 2:42 PM on February 28, 2007

If it's a rental, sure. I managed to squeeze 3 extra mpg out of a recent rental (some kind of fairly sporty 2-door Pontiac) by going into neutral on long (but gradual) downgrades on a recent mountainous trip. The fuel injectors don't inject a uniform flow -- it reduces as engine RPMs reduce. So if you have a nice long stretch to coast in, it can make a marginal difference. And you can use the momentum to gain acceleration on the upgrades without hitting the gas hard. I know, because I burned through 2 tanks of gas getting unbelievably bad mileage that was just pissing me off before I decided something had to be done.

Don't do it in the mountains unless you're a damn good driver, though. It's real easy to lose control at high speeds with no engine or gearshift braking, and you could end up overheating your brakes very quickly too.

I wouldn't, personally, do it on my own car's tranny. But then I wouldn't own an automatic if you paid me to drive it.
posted by fourcheesemac at 6:21 PM on February 28, 2007

Engine breaking on long inclines is still the state of the art as far as I know, toastchee, for mountain driving. It's not saving the brakes, but avoiding them getting so hot they fail and kill you. And engine braking gives you much more control over the car or truck.
posted by fourcheesemac at 6:23 PM on February 28, 2007

braking, not breaking, sorry
posted by fourcheesemac at 6:23 PM on February 28, 2007

Again - see here for fuel usage while engine braking - deceleration fuel cut-off.
posted by opsin at 10:00 AM on March 1, 2007

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