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Thread: coasting in neutral ?

  1. #1
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    coasting in neutral ?

    When I had a 5spd corolla, I'd let it coast in neutral to save a few drops of fuel and extend my MPG. The way the 5's automatic shifter is set up, it's easy to bump it into neutral from drive and coast a bit...

    However, I just read in the manual that one should not shift from drive into Neutral while going faster than an idle speed.

    Is this one of those things where they are warning you because some idiot will forget he's coasting in Neutral and get plowed by a semi while merging on the highway? or is there some danger to the tranny when moving from drive to neutral at speed?

  2. #2
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    '06 Mazda5 Sport MT, '09 Honda Fit Sport MT

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    It's probably more of an idiot thing. But the question is, is it even necessary? I was under the impression that most, if not all, modern cars shut off the injectors when coasting and no throttle. Thus you're not saving anything by shifting into neutral because the car's not using any gas anyway.
    2009 Honda Fit Sport 5MT
    2006 Mazda5 Sport 5MT

  3. #3
    That just happened GoFast's Avatar
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    Quote Originally Posted by doctorz View Post
    It's probably more of an idiot thing. But the question is, is it even necessary? I was under the impression that most, if not all, modern cars shut off the injectors when coasting and no throttle. Thus you're not saving anything by shifting into neutral because the car's not using any gas anyway.
    Right...if anything, you are actually using more gas that way.
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    BlackCherry06 BlackCherry06's Avatar

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    Auto transmissions, from my experience are set up in two different ways in regard to the fluid pump. One type pumps fluid whenever the engine is running (I've never seen one of these, but have been told they exist), and the other type only pumps fluid when the transmission is in reverse or drive. If yours is set up to run the pump only in drive or reverse, you're basically spinning your transmission without fluid being pumped through it. It's the same reason they tell you not to tow a car with the drive wheels on the ground if you have an automatic.

    In addition to that, if you leave it in drive and release the accelearator, the fuel injectors cut fuel flow until it slows to nearly idle speed. If you put it in neutral and coast, the injectors have to maintain the flow of fuel to keep the engine running. You're not saving a thing.


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  5. #5
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    2007 Mazda 5 GT 5MT

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    When you coast in neutral, the fuel pump has to inject gasoline into the engine to keep the engine idling. When you coast in gear and don't have your foot on the throttle, the fuel pump does not inject gasoline in the engine and the downhill momentum of the vehicle keeps the engine running, therefore you use less gas than coasting in neutral. It also saves your brakes since the engine helps keep your car from accelerating too much.
    2012 Mazda 3 GS-Sky 6MT Previously: 2007 Mazda 5 GT 5MT

  6. #6
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    Thanks, that's really helpful info.

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