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Automatic Transmission - Saveing Gas

  1. Apr 23, 2008 #1
    I have a 2005 Mazda 6, AT, 4 speed... I was wondering how much harm does it do to the transmission if you were to put the car into neutral just as you go down a hill then put it back into drive once the hill levels out? Of course this saves gas and allows you to increas speed. I used to drive an 80 something automatic and this allowed me to save an insane amount of gas. I could get 32mpg easly and I live in WV with turn after turn and steep hills. That old car didn't matter to me sense I had a back up and well really I couldn't wait till it died so I could get it out of the way. We have 4-5 cars laying around at any given time. That car was at the end of its life when we got it and that was back in 2000 and it's still running good. We take care of our cars! Right now my parents are using it while they work on thire regular car. So anyway, I have a new car with low millage and expensive parts!!! But of course its MPG on the back roads here sucks. It does great on the highway. It has taken me a while to learn to work the pedal so it won't shift every other second. It can be touchy at times... But anyway, i've given it one test and felt how smothly it can switch from neutral back to drive. And I got 33mpg on the back roads! When I normaly get 22mpg. I'm not about to take this into practice with this new car without some good advice!
     
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  3. Apr 23, 2008 #2

    Mech_Engineer

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    You should be fine, shifting into neutral will not have any damaging effects. Just be careful not to accidentally shift into reverse or park, then you can do damage to the transmission.
     
  4. Apr 23, 2008 #3
    But shifting from neutral to drive might reduce the estimated operation life, because of the impact at each occurence.
     
  5. Apr 23, 2008 #4
    Um, couldn't you have just said it might damage it...? Which in the long run would reduce its life... ... ... What I'm looking for is facts. Not that it might, because "how much damage" is the question I posted in the first place. I can save $500 or more a year like this, but thats not worth replaceing the transmission.
     
    Last edited: Apr 23, 2008
  6. Apr 23, 2008 #5

    brewnog

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    Modern cars with fuel injection actually use zero fuel on the over-run; ie supply is cut off when you 'coast' up to a red light, or down a hill. Putting the car into neutral will use fuel to keep the engine idling.

    So, to save fuel, keep it in gear, try and keep the engine operating close to peak torque, and just treat the accelerator like you're treading on an egg shell.
     
  7. Apr 23, 2008 #6
    Interesting, but I really don't see too many red lights around here... just trees and durt. Anyway, that may be true when going under 40 or lower. 'coasting' to a stop.... But I'm usally going 60+ down a long hill where you can't coast. I have experience in useing this method as I said above and it does save gas.

    I take two roads to get to places. One to get to town and one to get back home. This way I save gas no matter how I drive. But on the way back I can save the most becuase its nothing but down hill, miles long. Of course theres one or two times I can use it on the way to town as well.

    Good advice but it has nothing to do with my question...
     
  8. Apr 24, 2008 #7
    X2. Every fuel injected vehicle I know of (post-'86 at least) cuts power to the fuel injectors when engine vacuum increases while in gear, ie; coasting downhill without your foot on the and in gear. Fords EECIV manual specifically states this.

    It also applies to carbureted engines... you'll get the best gas mileage just keeping it in gear with your foot off the gas.
     
  9. Apr 24, 2008 #8
    Ok, so tell me how I am to go down hill going 65+ with my foot off the gas? It only slows down. It doesn't stay at its current speed and It doesn't increas speed.

    In my test of course I was more aware of my driving so I'm sure my gas millage increased form that alone. But in neutral I can go down one hill and up another down the next and so on and keep increasing speed. Decreseing my driveing time. So maybe I'm not saveing gas just going down a hill, but I do know I'm saveing it going back up another.

    This is very good information. But saveing gas is somewhat second to my orginal question how much damage will occur when you are driveing and you trow it in neutral and then back into drive?
     
  10. Apr 24, 2008 #9
    Well, first you did not ask specifically for the damage in your first post anyway, and second, the damage might come from the impact that suddenly the load is introduced to the engine while this impact is negligible when the car is stationary, thus you have to take into account this information about the wear. If you want numbers and which part fails first, then you have to calculate the fatigue and fracture properties. Because no one can estimate here how you drive. You can ask about statistics about how long does it take for a transmission to fail, then you think of an average number of shifts to neutral and drive in a casual drive and your current shift number with your method. Get a safety factor and do the math.

    My intuition says it is not worth it. Because a failed transmission will cover all the savings that you got by this method. Also other unpredictable things might happen and no one can answer this without including some speculation.
     
  11. Apr 24, 2008 #10
    Harm = Damage
    Damage = Harm

    Am I not right? That was like the second sentence in my first post.

    If the title is your problem I'm sorry I couldn't figure out how to put what I wanted in a few words. Of course I wasn't asking how many minutes the transmission would last under what ever driving still I use. I did give an example: "when the hill levels out then put it back into drive. You can use this if you wish. Maybe: "Put it in neutral at the top of the hill while coasting and then at the bottom of the hill when it levels out put it in drive while coasting?...

    So I am asking the one person in this world who may give it a shot and say it could cause serious damage and or it'll take a few years but of course thats not worth it. The others only said it might cause damage. But I know it does cause damage. It has to. But we are all here to learn and ask the questions that haven't been asked yet. If I don't get an answer I'm fine. But maybe someone will go out and figure it out. Maybe even get an idea and design something amazing. Of course "including some speculation" is fine. I don't think I'll find one of the people who designed my transmission and whatever else.

    But anyway, I'm the one asking the question. You seem to have the abillity to answer it, yet you don't.

    I know there is someone out there who can give a real answer. Maybe they design transmissions, or whatever. But for some reason I don't think I've found that person yet...

    Thank you for finally realizeing what my real question was!!!
     
    Last edited: Apr 24, 2008
  12. Apr 24, 2008 #11

    brewnog

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    So you can go down a hill in neutral, but you can't in gear with your foot off the accelerator?

    Cobblers.
     
    Last edited: Apr 24, 2008
  13. Apr 24, 2008 #12

    Mech_Engineer

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    It's called engine braking.

    Just shift into neutral if you desire, it won't do any harm to your tanny. A few people brought up the fact that you will be increasing the number of "shift cycles" on your transmission, but I seriously doubt it will be so many more cycles that you will significantly decrease the expected life of the shift assembly.

    Besides, your transmission is waranteed by the number of miles on it, not the number of shift cycles. If it fails before the warrantee is up, get it serviced for free. If it fails after, I doubt it was due to shifting into neutral to coast down long grades.
     
    Last edited: Apr 24, 2008
  14. Apr 24, 2008 #13
    Right... Where do you live? It seems the faster you go the faster it slows down. So I can't even go over the speed limit and exspect to reach the bottom going the speed limit. I've really never given it a good test. But It begins to slow as soon as you take your foot of the gas, and soon it goes below the speed limit. Now speed limits aren't really an issue. But if it takes you 2mins to get down the hill normaly, I think it might take ten or more if I wasn't useing the gas.
     
  15. Apr 24, 2008 #14

    brewnog

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    But if the transmission is in neutral then the engine is running, but not applying any force to move the car forwards.
     
  16. Apr 24, 2008 #15
    Yeah, just like takeing a toy car and putting it on a hill... it rolls down faster and faster, without any engine. You don't even have to push it. But my car is in motion when I put it in neutral so it can continue to move even on flat ground... What was you point?

    See where Mech_Engineer was talking about engine braking. This term can be more commonly found in raceing, manuals can down shift to quickly reduce speed with or without brakeing. But still the fact is the same the engine without gas adds to the moveing parts makeing it harder for the wheels to move. Where neutral will basicly disconnect the transmission from the engine.
     
    Last edited: Apr 24, 2008
  17. Apr 24, 2008 #16

    Mech_Engineer

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    But, if the transmission is in drive and the engine is producing less power than is required to overcome friction in the engine and transmission as well as air and rolling drag, your car slows down. This is called engine braking.

    Point is, shifting into neutral is fine. It's the same as if you pushed the clutch in on a manual. Your engine will just idle happily and the car will roll freely down the hill.
     
  18. Apr 24, 2008 #17
    Well, if you do think of it as a manual. Then you would think the computer would do a better job to match the RPMs to the transmission before connecting them... But then again the computer wasn't really designed to go from neutral to 4th while going 65mph... An automatic transmission is extremely diffrent from a manual... So would you say that it would do as good of a job as a basic manual would?

    Then if you take in account a sudden change in speed or load by going back up a hill when you choose to put it back in drive. I would perfer change into drive before the conditions changes suddenly, of course on flat ground before the next hill. But just to cover all corners, would you think it would have trouble with this. Maybe to the point of damageing it?
     
    Last edited: Apr 24, 2008
  19. Apr 24, 2008 #18

    Mech_Engineer

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    No one is going to be able to give you a 100% definitive answer. Some will think one way, some may think another, but in the end it depends on how much you trust the engineers that designed your car, and how much the savings are worth. If you want a 100% definitive answer, go find the engineering team at Mazda that designed your specific transmission and ask them.

    Automatic transmissions have a torque converter that does a lot in terms of reducing shock loads to the tranny. Still, automatic transmissions are extremely resilient, and as a general rule it is said that "anything you feel, your transmission doesn't." Your engine probably only jumps up a thousand rpms or so from idle to top-gear at 65mph anyway, that's no worse than downshifting from 4th to 3rd at 65mph, which the transmission definitely was designed to be capable of. I guarantee a full-throttle shift from first to second gear is a lot more stressful on your transmission than shifting from neutral to drive at 65mph.

    Shift back into drive before you press the accelerator, and you're fine. It seems to me that your are worrying WAY too much about this.
     
  20. Apr 24, 2008 #19
    No I'm not worryed about this at all. This is what I do, learn. I speak a couple languages, and have certifications in computer engineering, and much more. It's just what I do, question everything. I was just asking your oppion.

    Your right I did sound worried in that last post. I do know how a automatic transmission works an expected you to say what you said. So I chose to lean toward questioning it rather than agreeing. My oppion dosen't matter to me at all times.

    Oh, P.S. I suck at spelling in English... Thats what you get with your first language... lol
     
    Last edited: Apr 24, 2008
  21. Apr 24, 2008 #20
    It no doubt does. I once had an automatic AOD Mustang that started to slip, and whenever I would do a full-throttle shift, it would continue slipping until I let off the gas a bit. Popping it into neutral and back into drive again at freeway speed wouldn't make it slip at all. And thats while coasting on level ground.

    With the bit of experience I have working on older transmissions, i'd have to say it doesn't adversely affect the life. Then again, most of the damage i've seen is due to parts that were unable to handle the power at hand. Like Mech_Engineer stated, your best bet would be to ask a Mazda engineer. I doubt transmissions have really changed that much over the years though.
     
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