By on November 10, 2010

Ernest writes:

Hi Sajeev, first of all I enjoy your column very much. The Honda transmission post over in the “New or Used” column raised my interest in something I’ve always been curious about. I have a few  questions:

  1. I used to compensate for my lead foot by coasting down slight slopes or towards a red light in neutral (in my automatic.)  I used to drive a manual and I understand it can be dangerous to coast down a steep slope because the differential becomes free and so the rear wheels can turn in opposite directions, potentially allowing for a spin.
  2. I stopped the practice in my automatic after learning (from a usenet board years ago) that the multi-plate clutch has very small clearances in N and consequently leads to overheating if allowed to coast.  I’ve also heard that you don’t even want to idle at a standstill in N.  Is this true?  What the heck is N in an automatic intended for anyway, if you’re not supposed to use it?
  3. It was mentioned that you shouldn’t go into reverse without doing a complete stop.  Same idea?
  4. Is it better to stress the engine at low rpm in overdrive, or to stress the transmission at high rpm in D4 (or D3 or whatever)?

Sajeev Answers:

Let’s get right to it.

1. I am not an expert in FWD transaxles, but putting a transmission in neutral has no effect on the differential, that’s downstream of the situation. More to the point, the (non limited-slip) differential only spins in different directions when airborne, it’s physically impossible to make one wheel spin the other way on the road, as it is loaded by the inertia provided by the car’s weight and forward momentum. Too bad it doesn’t work that way, though. Would make for a lot of fun on icy roads!

2. That’s all incorrect, unless the vehicle is flat-towed (all four wheels on the ground, like what you see with RV towing) and the engine is not running. Neutral is perfectly safe to run forever, because the running engine activates the transmission’s pump, circulating fluid to everything. Saw that for myself when adding fluid to the freshly-rebuilt gearbox in my daily driver two years ago.

3. Yes, but not for that reason. The load placed on clutches with the violent engagement between forward/reverse causes the problem, the clearances between neutral are irrelevant. Pop the car in neutral while slowing down for the gear change to smooth out and speed up this process. This is a great way to multitask!

4. It’s a difficult answer, mostly because the variables in one’s terrain, transmission shift logic (be it old-school mechanical or today’s electronic stuff) and the amount of cargo come into play. Your answer may be irrelevant to others. What’s the safe bet? Let your modern car do what the engineers intended, the stock transmission shift logic is normally “geared” (sorry) to improving durability. But if you live in Colorado and climb a hill at 1000 rpm in top gear, better override the system for your own sake.

Whew! Off to you, Best and Brightest.

Send your queries to [email protected]. Spare no details and ask for a speedy resolution if you’re in a hurry.

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69 Comments on “Piston Slap: Transmission Talk, Debunked...”


  • avatar
    srogers

    Why coast in neutral?
    I can’t believe that you would burn noticeably less fuel this way than just taking your foot of the accelerator. The thought of coasting powerlessly gives me the willies.

    • 0 avatar
      nonce

      I was skeptical, too, especially since my RPMs did not seem to drop in neutral.
       
      But in actual tests, I use much less gas coasting in neutral than just with my foot off the accelerator.  I got more than the EPA highway mileage on a recent camping trip, loaded with gear, running the AC, by timing the use of acceleration versus coasting.

    • 0 avatar
      st1100boynd93

      In my 5-speed Focus I go beyond simply coasting in neutral: I actually turn off the engine entirely if I have a long coast up to a red light or even down a steep hill.  If I’m approaching a light, once the engine is off, I turn the switch back to on, then while still rolling at maybe 5-10mph, I put it in 4th and bump start the engine.  Works great.  Poor man’s stop/start technology.

      I’ve always done that same basic trick, plus coasting into a parking spaces and driveways when I can.  I don’t know if it saves much fuel (probably not), but it is sort of fun.  Feels like I’m getting away with something. 

      As for the safety argument about coasting in neutral, I don’t see the problem.  The brakes still work, the steering still works (no power assist, big deal).  Only takes a second to bump start from any speed if I need to accelerate.  Still too dangerous for you?  Put on a dress and take the bus.

      I’m a big motorcyclist, and one of the things that always bugged me was only being able to get into neutral from 1st or 2nd. The basic controls on a bike make more sense than on a car (the key controls are through the hands, not the feet), but it would be cool to access neutral at any time.  

    • 0 avatar
      Beerboy12

      oooeeer! engine off… no power breaks and no power steering. I would not like to be a passenger when a deer runs out in front of you… Still, good on ya for manual tranny and biking.

    • 0 avatar
      JuniperBug

      As I understand it, fuel injection systems of the last 20 years or so have automatically cut power to the injectors when the throttle is at idle and the car is coasting in gear. Therefore, being in high gear with your foot off the gas actually uses less gas (ie: none) than coasting in neutral (where of course fuel is being injected to keep the engine running). Any hill that’s long and steep enough to think about coasting down, is probably steep enough to not worry about the minimal engine braking slowing you down. In this mode you also keep all your accessories going, so no worries about power steering or brakes.

      If you have a car equipped with an instantaneous MPG meter, try it yourself. On the diesel Renault Master I drove in Europe, it would read “0L/100 km” whenever I’d close the throttle while coasting in gear.

  • avatar
    tankinbeans

    With regards to the question about coasting downhill in neutral I think it’s best to leave the car in gear. I’ve read in other forums that if you coast in neutral the engine has to work in order to keep the crankshaft spinning and thus uses more gas, but if you were to coast in gear down a hill the wheels, locked to the transmission, will spin the crankshaft and the fuel injectors will shut themselves down. If you have an onboard mileage computer, or a detachable unit, you can see when this happens. In my Pontiac if I am going downhill in gear the instant economy readout will read 99.9mpg indicating that no extra fuel is being injected into the engine.

    • 0 avatar
      Russycle

      It’s true that many modern engines shut off the injectors when you coast in gear and let inertia turn the crankshaft.  However, I find that I can coast much further in neutral, as there’s no friction from the drivetrain to slow you down.  Just depends on the situation as far as which is better.

    • 0 avatar
      Mike66Chryslers

      Not surprising that you will coast further in neutral than in gear… it’s called “engine braking”.  If you want to slow down even faster, manually shift into a lower gear.

    • 0 avatar
      johnny ro

      It takes more power to spin the engine fast with fuel cut off than it does to spin it slow with fuel on.
      Overall a small difference to range on a tank.

  • avatar

    FYI: every car I’ve driven with an instant MPG gauge reads higher when coasting to a stop in neutral. I will occasionally do this when coasting to a stop in the flat concrete jungle of Houston, where it actually can be performed without fear of causing an accident.
    Why the better MPG? Because you keep your MPH higher with no load on the powertrain.  Higher MPH combined with an idle below 1000rpm (not possible to go that low when in gear) makes for 10-20 MPG improvements in neutral vs. drive.

    • 0 avatar
      jmo

      I’ve driven with an instant MPG gauge reads higher when coasting to a stop in neutral.

      When you coast to a stop in gear the gauge reads 99.9 or infinite MPG as no fuel is being consumed.  If you are in neutral it will calculate the MPG based on the fuel being used to idle the engine.

    • 0 avatar
      Beerboy12

      I had a Renault Clio III that reads 0 MPG (L / 100km) When coasting down hill in gear. Push in the clutch and the meter would jump to idling MPG. I expect it shut down fuel supply while coasting in gear. It was a 1.6l four pot that is capable of well over 120mph and could be quite heavy on gas mileage when driven in anger so I guess it needed a few tricks to keep the MPG down. It is a good car that would work well here in the US market.

  • avatar

    I felt like asking much the same questions, as lately I’ve been using N to coast to a stop and idle at red lights, because I do perceive a benefit in fuel economy and also because my engine mounts are wearing thin and there’s less vibration at idle in neutral.
    When it comes to answering question 4, I manually override the manufacturer’s shifting logic under certain driving conditions. In a reputable auto column, I read that frequent gear changes causes wear and tear that will shorten the life of an automatic tranny. I have a plain jane 4-speed and I often set it to 3 for driving in residential areas (where speed limits are 30 mph or less, all-way stops are frequent, and other traffic calming measures are present). My torque-converter will lock-up in gear 3 or 4, and so it revs quite nicely in third gear up to 30 mph. I also perceive this as improved fuel economy as the manufacturer’s logic would set it to 4th with the torque converter slipping up to 40 mph.

    • 0 avatar
      YYYYguy

      Our 02 Forester is a beast in (D).  It vibrates like no other at idle, but shifting to neutral returns the drivetrain back to “smooth”.  Do you think the motor mounts need replacing?

    • 0 avatar

      Could be, but there are a lot of factors that contribute to vibration at idle. In my case, engine mounts are a contributing factor (under observation by an experienced mechanic). My tailpipe & muffler were recently replaced (along with the mounting brackets) and I noticed an improvement at idle, that was an unexpected benefit. Maybe you want to submit your own Piston Slap to Sajeev?

    • 0 avatar

      BTW, I just did a short trip in a well maintained ’87 Volvo 740 Turbo stick… it’s been at least five years since I had the opportunity to row my own gears. How I miss the pleasure of rev matching and downshifting… I think I play around with my automatic tiller a bit more than I should because my right hand needs to be doing something whenever I drive.

  • avatar
    Redshift

    Good little article over on Popular Mechanics about it:
    http://www.popularmechanics.com/cars/how-to/repair/coasting-in-neutral-fuel-economy
    The jist is, you burn more fuel coasting in neutral than not.
    The on-board MPG gauge isn’t measuring actual fuel flow but is using sensor readings vs. the fuel map etc. to figure out what the fuel flow should be and reports that value. However, the injectors are shut off, so that throws off the reported values.
    Now, on a non-fuel injected car (not many left) that goes out the window.

    • 0 avatar

      Per article:
      The algorithm the trip computer uses is not based on how much fuel is actually consumed, but on some calculated value based on airflow past the mass airflow sensor, manifold vacuum and engine rpm.
      Not on EEC-powered Fords, that I know for sure.  The computer reads injector duty cycle (on/off) multiplies it by lb-hr rating of the injectors to calculate fuel flow. From there the speed sensor adds the miles to create the MPG. I learned this the hard way by adding 24lb-hr injectors on a (19 lb/hr) Foxbody Lincoln Mark VII without changing its value in the engine computer (via chip). The car always got high 20s in MPG, it was always wrong by 5+ MPG.
      Then I got a twEECer and saw how Ford made that work.
      So I seriously question that article.

    • 0 avatar
      Redshift

      Interesting info about the Fords.  Didn’t know they worked that way.  But, I’m wondering if that’s not more the exception than the rule.  Good question though.
      With a modern FI system, where the injectors are shut down while the car is rolling in gear (which I’ve seen a lot of examples showing that it does… did Ford just not figure that out?) how can you burn less gas than none?

    • 0 avatar

      No clue how many other systems are like the Fords. Or if all OBD-II products calculate the same way, my knowledge might be a handicap. No matter, EEC-IV came out in 1984, and I am 99% sure it shut off the injectors while coasting too.  None of this is especially cutting edge, any processor smarter than ATARI PONG can make a modern engine run flawlessly.
       
      Re: mileage, you’re forgetting the “M” in MPG. When you don’t have drivetrain loss from the transmission/axle being defeated in neutral , you retain higher speeds for longer time.  Plus, any EFI system barely spits any fuel out at idle. 800 rpm with the gas on isn’t that much different than 2000rpm while shut off.

    • 0 avatar
      bam210135

      “cars don’t handle well in neutral during sharp cornering maneuvers when the engine isn’t connected to the drivetrain.”

      How does the engine/transmission have anything to do with cornering maneuvers? Im pretty sure that steering or suspension doesn’t change in neutral. Yes the drag of the drivetrain isn’t there, but that shouldn’t affect the turning ability.

    • 0 avatar
      Redshift

      Good points again.  It seems to really be a big “it depends” situation.  You’d need to compare the rolling resistance of the drive train in question vs. the maps the the FI system uses in that situation vs. the geography you are rolling across etc.
      Personally, for any small savings there might be in some situations, it probably isn’t worth it. But, it does sort of define the idea of YMMV. Also, I just might not have long enough/steep enough hills on the highways where I live to see the value.

    • 0 avatar

      I will have to retract my statement about the PM article. After further discussions with people far smarter than I, turns out everything changed when we went to government mandated OBD-II programming in 1996.
      My bad.I should stop living in the mid-80s.

  • avatar
    findude

    If you want to play with your transmission, get a vehicle that has a manual transmission! The automatic transmission was given that name for a reason.  I can only think of two reasons to drive (forward) in anything other than D: a long, steep downhill so that the transmission can hold the car and you can reduce brake wear, and if you are towing a trailer and the owner’s manual recommends that you select D3 (for example) to prevent torque converter lock-up.

    • 0 avatar
      ott

      +1. Let the transmission do its thing and just enjoy the ride.

    • 0 avatar
      Shane Rimmer

      The 5-speed automatic in my Altima automatically starts downshifting on steep downhill runs, so I just leave it in drive and let the transmission do its thing.

    • 0 avatar
      trk2

      I can add one more reason.  On Ford transmissions “2” used to be second gear only.  So not only could you force the downshift to second gear for engine braking, but you could force your vehicle to start out in second gear for driving in snow or mud.  This was true for Ford truck transmissions through the mid-90’s, but I don’t know about the current Ford transmissions.

  • avatar
    HoldenSSVSE

    Many years ago, when automatic transmissions were mostly 3-speeds I read a very similar question 4 posed in one of the car mags (Road & Track, Car & Driver, etc. etc.) looking for an answer.

    For downshifting on a hill the answer was real simple.  Which costs more.  Brake pads and potentially rotors, or a clutch if you have a manual, or a full on transmission if you have an automatic.

    Modern transmissions, even in crappy cars are computer controlled, with multiple programming curves that can identify driving conditions and choose accordingly.  Just as wisely answered above, unless your loafing up a hill at 1,000 RPM and the car isn’t shifting, or your screaming at redline flying down a hill, let the tranny decided on its own.

    • 0 avatar
      potatobreath

      Manually downshifting for steep downhills is sometimes necessary to avoid cooking the brakes. I don’t think most of the cars on the road have hill descent logic in their automatic transmissions yet, since I see people barrelling down steep hills with their brake lights lit up most of the way. For manual transmissions, I rev match my downshift to minimize clutch wear.

    • 0 avatar
      ivyinvestor

      @Potato:
       
      I think many autos actually do have descent logic…Honda had “Grade Logic” on the 1995 CR-V (for sure, upon introduction) and EX Accords before that (Civics a little afterward, if memory serves). I think Subaru introduced it around the same time, and I remember seeing ads for programming schemes akin to these in Toyotas and Nissans within a few years, if not around the same time.
       
      I believe part of what you’re noting is that some folks don’t understand that they should modulate their throttle position…Uphill, more; downhill, less, all else equal…
       
      Good point re: rev matching, though…I recently tried to convince a manual shift-lover in his late fifties of the benefits – and enjoyment – of it…As person who has driven some sweet wheels, I expected him to already do it. He didn’t, and he had no interest in it, either…

  • avatar
    Sinistermisterman

    Coasting in neutral down a hill in a manual is a dumb idea. During a teenage camping trip I was following a friends old Volvo down one of the windy mountain passes in the Lake District in the UK. He decided to save fuel on the way down and coast – at the bottom his brakes were absolutely cooked and he couldn’t stop. He eventually had to use a dry stone wall to slow him down so he didn’t run over a bunch of hikers. I on the other hand used low gears to slow me down and bingo! My brakes weren’t cooked by the time I got to the bottom.

  • avatar
    bootsfirst

    On coasting in N- the mpg gains aren’t worth the risks, why leave yourself with so much less control.

    This is kind of off-topic but point # 4 reminded me of something I’ve been wondering about:

    I recently learned to drive stick and my car has a sort of dead zone it hits when I have to slow to 10-15 mph for a speed bump or similar-  if I stay in 2nd i’m at 1000 rpm and the engine feels like it’s in molasses (lugging I assume) but downshifting to 1st (which I know you’re supposed to avoid) makes for a rough shift and seems like a recipe for fucking up my clutch or tranny.

    What’s the appropriate strategy?  I usually stay in second and ease back into the gas gently.

    • 0 avatar
      srogers

      Why avoid downshifting to 1st?
      It’s not any more of a problem than downshifting to any other gear. If you want to be gentle, try to match your revs, but that’s really only necessary if you’re driving more aggressively than you probably are slowing down for a speed-bump.

    • 0 avatar
      Sinistermisterman

      Ride the clutch gently as your engine begins to struggle – don’t downshift – and as you get going again, bring your foot back off the clutch. No ‘lugging’, no rough downshifts… just smooth and controlled.

    • 0 avatar
      mikey

      @ sinister….I agree with your advice. But having learned on a “three on the tree” I can still hear my Dad “if you not going to use the clutch take your F—KEN foot off it”

    • 0 avatar
      Demetri

      “On coasting in N- the mpg gains aren’t worth the risks, why leave yourself with so much less control.”
       
      You only lose the ability to immediately accelerate.  You can still brake and handle just fine.  There is no situation that will require you to immediately accelerate to avoid a collision, etc.  If you have time to accelerate past a situation, you have time to put it in gear first and then start accelerating, and if you’re driving manual you’ll do it faster, because you’ll chose a lower gear than the one that you would have been in had you not been coasting.

    • 0 avatar
      Sinistermisterman

      @mikey
      Actually that was my constructive criticism to my father! He has had a string of company car and routinely burns the clutch out ever 20,000 miles by zooming down the motorway with his foot resting on the clutch. Ah well, he doesn’t crash as many of his cars as I do mine.

  • avatar
    65corvair

    D is for forward, R for reverse. That simle, don’t like it, you need a manual transmission.  That’s the point of an automatic.

    Let the brakes slow the car down hills and stop the car before going from reverse to drive.  That is their job.  If it’s a 1936 Chevy, then shift into low, but modern brakes your not goin to fade them. 
    On anohter note, my Corvair’s power(less)glide was designed to go from reverse to drive full throttle 100 times without failure.  Never tried it but it still works fine. 

    • 0 avatar
      nikita

      Way too many people do this going down California mountain roads. You see brake lights on the whole way down and smell the burning linings. Many automatics are designed to coast in OD, and that is dangerous on steep downgrades. Modern brakes do not have infinite fade resistance. Momentum is still converted to heat.

    • 0 avatar
      windswords

      We experienced this in our vacation to Arizona recently. I knew better and shifted our Aveo rental into 2nd gear as we descended a mountain. We stopped for a moment at place to park and take in the scenic view and another rental, a Chevy HHR, pulled in. You could smell the brake linings from 20 feet away.

  • avatar
    TexasAg03

    In many states, it’s specifically against the law to coast in neutral.
    From the Texas Transportation Code:
     

    Sec. 545.406.  COASTING.  (a)  An operator moving on a downgrade may not coast with the gears or transmission of the vehicle in neutral.(b)  An operator of a truck, tractor, or bus moving on a downgrade may not coast with the clutch disengaged.

  • avatar
    John Horner

    The “OBDII killed deceleration fuel cutoff” idea appears to be more urban legend than fact.
    Have a look at the paragraph entitled: “Deceleration Fuel Cut” in the Toyota service literature under the OBDII Fuel System #2 chapter:
    http://www.autoshop101.com/autoshop15.html
    http://www.autoshop101.com/forms/h43.pdf
    “To prevent excessive decel emissions and improve fuel economy, the ECM will not open the injectors under certain decel conditions ….”
     
     

  • avatar
    Mike66Chryslers

    Regarding #4 and “stressing” your transmission:
    “Back in the day”, Drive was the gear where the driveshaft speed was the same speed as the engine crankshaft.  Consequently, Drive used the least gearing in the transmission so generated the least heat.  Later transmissions added Overdrive, a gear in which the engine crankshaft turns faster than the driveshaft.  One problem was dumb people that put their newer cars in Drive because that’s what they selected on their older cars.  (Not kidding, I have met these people, driving on the highway in Drive with the engine screaming.)  So now we call the gears D, D3, D4, whatever.  I’m going to use the terms Drive and Overdrive, and you can figure out for yourself how that relates to your own car.
    Two things that “stress” your transmission are gear changes and heat.  Ye olde 3-speed automatics coupled to torquey V8s seemed less failure-prone than today partly because, once the cars got moving, the trans tended to shift up into Drive and stay there unless you mashed the gas pedal; the engine’s torque was adequate to accelerate with traffic without the need to downshift.  Also, since Drive basically couples the engine to the driveshaft with a minimum of gearing, it produces less heat.
    Modern automatics have Overdrive (or multiple overdrive gears!) and lockup torque converters coupled to smaller, less-torquey engines and the electronics are trying to eek-out the highest mileage possible.  When driving around town, they tend to hunt through the gears as you speed-up and slow down.  Do you want to do your transmission a favor?  Shift down from Overdrive to Drive in the city.  Shift back to Overdrive when getting on the highway.

  • avatar
    John Horner

    ” Ye olde 3-speed automatics coupled to torquey V8s seemed less failure-prone than today partly because, once the cars got moving, the trans tended to shift up into Drive and stay there unless you mashed the gas pedal; the engine’s torque was adequate to accelerate with traffic without the need to downshift.  Also, since Drive basically couples the engine to the driveshaft with a minimum of gearing, it produces less heat.”
    No exactly. Ye Olde automatics used non-lock-up torque converters and the engine speed would always at least slightly exceed the drive-shaft speed due to the imperfect nature of a fluid coupling. Lock up torque converters add a clutched mechanism which is able to lock the input and output shafts of the torque converter together and thus take the fluid coupling effectively out of the picture. The old way resulted in more heat in the transmission because those losses in the torque converter showed up as heat.
     
     

    • 0 avatar
      jj99

      Very good.  You must be a mechanical engineer.

    • 0 avatar
      Mike66Chryslers

      I’ll give you that, John.  Lockup should keep the temps lower and improve fuel economy, which I overlooked.  Fortunately, transmissions with a locking converter will lockup in both Drive and Overdrive.
       
      My basic premise is still true: Transmissions in new cars shift a lot more than older ones, including locking and unlocking the converter.  Downshifting to Drive in the city saves wear-and-tear on your transmission.

    • 0 avatar
      John Horner

      “Very good.  You must be a mechanical engineer.”
      Close, electrical engineer. But I’ve been working on cars since I was in fourth grade and had to take a few physics and mechanical engineering courses along the way :). I was one of the weird EE students who was excited about taking statics and dynamics.
       

    • 0 avatar
      Mike66Chryslers

      That pretty much describes me too.
       
      BTW, the discussion of lockup converters is basically an aside to my post. The point I was trying to make by discussing the gears was that Drive will produce less heat in the transmission than higher (Overdrive) or lower gears, because less gearing is involved.  The same is true regardless of whether the transmission has a lockup converter or not.
       

  • avatar
    pb35

    While we’re on transmissions, I would like to pose another question. I recently had my trans flushed on my 2004 G35 at 53k per the dealers recommendation. Since I got it back, it shifts “funny.” Not slipping or anything, but it just doesn’t feel right. It just feels a bit sloppy now.

    I asked my father in-law (former mechanic) about this. He said that it shouldn’t have been flushed if I was over the 30k mark. Is this correct or just an old wives tale? I am ready to pour some sawdust in the trans and trade it! Seriously though, it’s fine, it just feels weird.

    • 0 avatar
      John Horner

      Several things could be going on. One, the dealer may well have used an incorrect transmission fluid and/or unapproved additives. Second, the flush machine may well have used a solvent clean procedure, which most auto manufacturers caution against. Finally, the vehicle may need to relearn its transmission shifting calibration due to the different viscosity and other characteristics of the fresh fluid as compared to the worn stuff. Typically an appropriate scan tool plugged into the car can instruct the transmission computer to go back to baseline calibrations and then to re-adapt itself.
      A dealer service department SHOULD be aware of all these concerns and SHOULD have worked accordingly. But, many don’t and don’t :(.

      Finally, there is nothing wrong with CORRECTLY changing out the automatic transmission fluid when it is done correctly. There is an urban legend floating around which says that the transmission fluid should be left alone until failure. Simply not true.
       

    • 0 avatar
      nikita

      Is it possible that the dealer used a “universal” fluid instead of the exact type Nissan used as factory fill back in 2004? ATF’s differ mainly in frictional characteristics. Also, it may be that shift quality changed gradually over 53K and you didnt notice, so the new fluid may have restored much of the original “feel”.

  • avatar
    TR4

    Question #2:  Three good uses for neutral are:

    When parking on a hill, it is better to shift into neutral, set the parking brake, then shift to park.  This prevents placing a load on the parking pawl and the resultant difficulty of shifting out of park.  Some trannys are worse than others for this.
    If the engine stalls while the vehicle is moving, the starter can be engaged without stopping the vehicle if you shift into neutral.  This was probably more useful in the days of carburetors which could ice up or have flaky automatic chokes.
    If the engine runs away due to accelerator pedal jamming shifting into neutral may help save your hide.

    Question #4:

    Usually the best fuel economy is obtained with lower rpm and higher throttle.  However when going uphill pulling a trailer you will likely go faster by downshifting and allowing the engine to rev higher and generate more power.  If you notice the transmission shifting frequently when going up a long hill it would be better for the tranny to put it into the lower gear and avoid all the shifting.

  • avatar
    blowfish

    Do you want to do your transmission a favor?  Shift down from Overdrive to Drive in the city.  Shift back to Overdrive when getting on the highway.

    When u drive on some hilly terrain, is better to lock out the OD, let her run on D3, shifting in & out is going to create issues down the road. Unless u going to trade or get rid of the car in a short time.
    One old guy didnt let his ford diesel truck to shift into OD while driving in the city, as reason if the RPM too low just barely enuf then it can put too much stress onto the trans.
    I usually like to adjust the shift point in my cars to higher RPM so it wont shift into high gear prematurely. Plus being diesel u get more torque by letting the engine to run on higher RPM.

    If u coast on neutral going downhill u better have some idea how much brakes is it going to use, there was a case of some idiot pulling a Uhaul trailer keep using the brakes on a steep downhill ended up burnt up the brakes and caused a severe fatal accident! He should have been using the 2nd instead of using the brakes all the way down hill. That was the famous Coqhuhalla hwy, coming back to Vancouver , it has a long down grade very hard on tranny, rad ,cooling , A/C systemgoing up and just as hard coming down.

  • avatar
    DC Bruce

    I think the answer is, it depends.  As cars get more and more sophisticated, they do a better and better job of optimizing everything.  For example, my ’02 Saab 9-5 with an autobox, essentially freewheels between about 30 mph and about 50.  You can tell this, by looking at the tach when your car is coasting.  At super-low speeds the tranny is always engaged, which improves driveability, since the car slows down immediately when you take your foot off the gas.  And, I suspect, for the same reason, at speeds over 50 the tranny doesn’t disengage.
    OTOH, my Honda Pilot doesn’t seem to show this behavior.  It seems to be engaged just about all the time, although may disengage when engine speed falls below 2000 rpm.
    Shifting an autobox into neutral while stopped does save fuel.  With automatic idle control, the engine works harder against the torque converter to maintain the preset RPM with the car stopped.  This also heats up the tranny fluid in heavy traffic.
    Re fuel shutoff while coasting, I’m not sure whether all cars are the same.  My ’92 SHO quite definitely shut off fuel while coasting, with a fully warmed-up engine.  I’m not sure about any of my newer cars, either automatics or manuals.  I don’t know that the mileage readouts can be trusted on this.  It may be that even with a little fuel going to the injectors, a coasting car reads MPG outside of the display range of the readout.
    The old 2-cycle engined Saabs did have freewheeling, and I believe it was not uncommon in US cars of the 1940s.

  • avatar
    blowfish

    Lock up kicks in when going at a higher speed, as the designer knew the slipping can be eliminated so it runs at a higher ratio.
    Yes it will decrease heat from the torque conv.
    I really dont know if most cars have enuf cooling capacity with the stock trans oil cooler. As u run thru the rad the heat kind of keep passing from 1 hand to the other hand! If u going to own the car for a little while longer an external cooler is not going to hurt. U also take some of the heat load from the rad too.

  • avatar
    Ion

    My 4R70W’s days are numbered, if I don’t shift into neutral at a stop or coasting I can actually feel the transmission cycle through gears and my fuel economy takes a nosedive.

  • avatar
    AICfan

    My friend tells me that the transmission in his mom’s new Escape has a neat trick:  If you set the cruise, it’ll actually downshift automagically on a steep hill to engine brake and maintain speed…

  • avatar

    I do not know how to drive an automatic. That is why I drive a manual.
    No mention of double clutching, heel and toe? Has everyone forgot how to drive a manual.

  • avatar
    Moparman426W

    John, you are correct regarding the lockup converter/less heat issue.  However when an automatic shifts between gears, up or down, it also builds up heat due to the slippage that takes place during the shifts. And shifting between neutral and drive in an automatic causes wear due to the engagement and disengagement of the clutches, so people that coast downhill in neautral with an automatic better be saving up for a rebuild.

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