Freewheeling - Mercedes Benz SLK Forum

General Discussion Topics must pertain to all 3 generations of SLK - R170, R171 and R172

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#1 Old 03-01-2014
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Freewheeling

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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#2 Old 03-01-2014
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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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Last edited by Woolly; 03-02-2014 at 11:21 AM.
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#3 Old 03-01-2014
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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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#4 Old 03-01-2014
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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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#5 Old 03-01-2014
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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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#6 Old 03-02-2014
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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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#7 Old 03-02-2014
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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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