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Discussion Starter #1
I live in a very hilly area. In my old cars, which were manual gearchange, I would depress the clutch when going down a large hill (freewheeling as we call it in UK).

Last week I tried this in my new SLK by pushing the gear lever into 'N'. Hey presto, no revs and freewheeling.

You would be surprised how this helps fuel economy. Remembering the place I live of course.

So my question is: does it do any damage to the auto gearbox when I slip into 'N'. Then when I level out I just knock it back into 'D'.
 

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Sadly Woolly has passed away
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You are advised not to tow a rwd automatic car with the rear wheels on the ground. 'Freewheeling'/coasting will have the same effect.

If you look at the price of a gearbox rebuild, compared with a gallon of petrol (or diesel!! ), I wouldn't take the chance.
 

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Premium Member 2006 SLK350
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Neither is recommended...pushing in the clutch at speed for prolonged periods causes excessive wear on the pilot bearing.

An automatic is essentially a hydraulic pump, by putting it in N you're decreasing the pressure of the pump. I'm not sure, but I'd be guessing that MB and all the manufacturers are very keen to increase gas mileage and minimize any drag in the trans while going downhill.
 

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Also, as far as I understand, it is even counterproductive to put it in neutral when going downhill on modern cars. It consumes more fuel, not less. If you let go completely of the gas pedal and the rpm is higher than idle (because a gear is engaged), the engine doesn't get any fuel at all. If you on the other hand put it in neutral, it does need a small amount of fuel to keep it running at idle speed (friction, alternator etc.). And you have to use the brake slightly more to burn off the corresponding energy.
I have read that some really modern cars even try to postpone charging the battery until these engine brake periods when the energy is free. Putting that kind of car in neutral would sabotage that fuel saving too.
 

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as mhslk said. you use more fuel keeping the engine ticking over when in neutral than the NO fuel when the engine is being pulled along by the car/gearbox. (when your throtle is not pressed)
this is true for electronic fuel injection vehicles. older carb vehicles do use a small amount of fuel and 2 strokes must keep an open (even if only slightly) when going downhill for a long time.
 

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Registered 2006 SLK350
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Don't do it Skinless.

It does say not to in the owners manual, plus you have reduced control in neutral.

As others have said, taking your foot off the accelerator is the best way of saving fuel. Our cars do not have an instant MPG readout on the computer, but my wife's auto 207 does - it says 999 MPG in similar circumstances.
 

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Discussion Starter #7
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Thanks guys very interesting. Im no engineer but i just assumed that when in N and the revs dropped so would the fuel consumption. But as you all seem to say its counter productive and may have consequences. Auto is much less controllable than manual I guess.
 

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Registered 1998 SLK230
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Trunk drivers call coasting in neutral 'angel gear' . Spend too much time in angel gear and you will see the angels. You have no control over a car when coasting . It's very dangerous.
 

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Premium Member 2018 SLC43
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I agree that fuel injection cars shut off the fuel when the car is going down hill..

Though the R172/SLC's have "Sailing Mode" on the new 9G Auto box - But never tried it....!!
I have read that some really modern cars even try to postpone charging the battery until these engine brake periods when the energy is free. Putting that kind of car in neutral would sabotage that fuel saving too.
That is correct, My Facelift R172 only charges the battery when slowing down....
First the mpg rises, they it charges the Battery:
571322


What is interesting, is that after a 2500 miles/90 hours of driving down to Alps/Italy and back, My R171/350 would be fully charged at 12.7 volts. But My R172 wasn't charging at all at a steady 130kmh, even on the down hill sections of the French Peage... This was reflected the day after We got back & was surprised how low the battery voltage was...!! :oops:
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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Trunk drivers call coasting in neutral 'angel gear' . Spend too much time in angel gear and you will see the angels. You have no control over a car when coasting . It's very dangerous.

Col


fyi


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