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E90/E91/E92/E93 (2006 - 2013)
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  #1  
Old 11-29-2012, 12:28 PM
unrealii unrealii is offline
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6MT - Coasting in neutral hurts mpg?

Yesterday I started thinking about this while stuck in rush hour. Many times while driving I will hold down the clutch to let the car coast when engine power is not needed. However I remember reading something that these engines can cut fuel when coasting. Which led me to believe coasting in neutral is wrong:

1) The car has to use gas to idle the engine while in neutral
2) The car can cut fuel when coasting and in gear as the wheels are spinning the engine

My unprecise anecdotal tests barely confirmed this to be true

What are your thoughts?
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  #2  
Old 11-29-2012, 12:34 PM
jburke4689 jburke4689 is offline
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You are correct but the difference should be inconsequential. You won't be able to notice it when calculating mileage.
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  #3  
Old 11-29-2012, 12:45 PM
gpburdell gpburdell is offline
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Quote:
Originally Posted by unrealii View Post
Many times while driving I will hold down the clutch to let the car coast when engine power is not needed.
My thoughts are that you're not really in neutral (rather, the car may not think you're in neutral) and you're putting additional wear on your throw-out (thrust) bearing. If you're going to go for Neutral, put it in neutral and don't ride the clutch.

As for fuel economy coasting vs. neutral, technically you'd save fuel coasting vs in neutral, but I doubt you'd be able to discern any real difference in real world usage. That said, clutch and throw-out bearing wear may or may not make a difference as well.
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  #4  
Old 11-29-2012, 12:58 PM
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SD Z4MR SD Z4MR is offline
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Regardless of whether there is any savings in fuel consumption or not, coasting in neutral in California is illegal, at least specifically on a downhill grade: http://www.dmv.ca.gov/pubs/vctop/d11/vc21710.htm. It is also illegal in many other states.

The DMV considers coasting as "unsafe". Here's an excerpt from a section on the driving test: "As you approach the turn: ...Slow down smoothly, change gears as needed to keep power, but do not coast unsafely. Unsafe coasting occurs when your vehicle is out of gear for more than the length of your vehicle.": http://www.dmv.ca.gov/pubs/cdl_htm/sec13.htm.

Coasting in neutral is considered unsafe because in an emergency situation you will not be able to respond as quickly if that response could include accelerating away from the emergency situation. Probably not worth the negligible gas savings, if any.
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Last edited by SD Z4MR; 11-29-2012 at 12:59 PM.
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  #5  
Old 11-29-2012, 12:59 PM
ctuna ctuna is offline
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In the 328 there is and instant mpg gauge

When you take the foot off the gas or coasting down a hill it goes to 50 mpg +.
For sure if you are going down a hill an just coasting in neutral you are wearing out
your brakes.
Would you coast a little further in neutral ? maybe
If it was a big gas savings thing wouldn't it be incorporated in all cars especially now?
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  #6  
Old 11-29-2012, 01:05 PM
unrealii unrealii is offline
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Quote:
Originally Posted by jburke4689 View Post
You are correct but the difference should be inconsequential. You won't be able to notice it when calculating mileage.
I was using the mpg # on the dash. It probably wont yeild huge savings, but knowing that my epiphany is correct is good to know.
Quote:
Originally Posted by gpburdell View Post
My thoughts are that you're not really in neutral (rather, the car may not think you're in neutral) and you're putting additional wear on your throw-out (thrust) bearing.
I totally forgot to consider the TO bearing. I wont ride the clutch like that from now on.
Quote:
Originally Posted by SD Z4MR View Post
Regardless of whether there is any savings in fuel consumption or not, coasting in neutral in California is illegal
Thanks for the input. I am well aware of that. Technically I'm still in gear. I always do this incase the engine should stop (very unlikely) but since the brakes are powered off the engine, I was always in a gear whether engaged or not for safety reasons except when stopped.
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Last edited by unrealii; 11-29-2012 at 01:06 PM.
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  #7  
Old 11-29-2012, 01:16 PM
Phil325i Phil325i is offline
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Coasting does not save fuel. When coasting your are burning fuel as the engine idles. If you slow down in gear (foot off the gas) the engine burns no fuel as the ECU cuts off the supply on overrun.
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  #8  
Old 11-29-2012, 01:16 PM
gpburdell gpburdell is offline
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Quote:
Originally Posted by unrealii View Post
since the brakes are powered off the engine
Huh? If the engine quits you might lose your power-assist, but your brakes should still work perfectly fine. You might just need to use a little more muscle, but the core system is still mechanical. Same for steering.

Last edited by gpburdell; 11-29-2012 at 01:17 PM.
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  #9  
Old 11-29-2012, 01:21 PM
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SD Z4MR SD Z4MR is offline
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Quote:
Originally Posted by unrealii View Post
Thanks for the input. I am well aware of that. Technically I'm still in gear. I always do this incase the engine should stop (very unlikely) but since the brakes are powered off the engine, I was always in a gear whether engaged or not for safety reasons except when stopped.
Sorry, but no. You are not "technically" in gear if you have the clutch in. Additionally, if you are coasting by holding down the clutch you are also putting unnecessary wear on the throw-out bearing. This is why it is universally suggested that you put the car in neutral and release the clutch when stopped at a light.

Also, brakes are not "powered off the engine". Brakes are a mechanical device with hydraulic assist. The hydraulic pump used in the brake system is powered by the engine. If your engine dies you can still stop, although it will be a lot harder to press the brake pedal.

ETA: the above poster beat me to it, apparently on both counts!
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Last edited by SD Z4MR; 11-29-2012 at 01:23 PM.
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  #10  
Old 11-29-2012, 01:24 PM
unrealii unrealii is offline
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Makes sense. Thought I was doing good, but apparently not.
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  #11  
Old 11-29-2012, 02:22 PM
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floydarogers floydarogers is offline
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Quote:
Originally Posted by SD Z4MR View Post
...
Also, brakes are not "powered off the engine". Brakes are a mechanical device with hydraulic assist. The hydraulic pump used in the brake system is powered by the engine.
This is not correct. Although a few braking systems (old BMW 7-series) use a hydraulic pump, virtually every one uses vacuum from the engine manifold to drive the brake assist diaphram. There is a small reservoir that runs out after a stop or two (whereupon the brakes still work, but you have to "stand on them".)

Even my 335d uses vacuum from an electric pump (as diesels don't have a throttle to create vacuum.)
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  #12  
Old 11-29-2012, 02:42 PM
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cwinter cwinter is offline
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Quote:
Originally Posted by SD Z4MR View Post


The DMV considers coasting as "unsafe". Here's an excerpt from a section on the driving test: "As you approach the turn: ...Slow down smoothly, change gears as needed to keep power, but do not coast unsafely. Unsafe coasting occurs when your vehicle is out of gear for more than the length of your vehicle.": http://www.dmv.ca.gov/pubs/cdl_htm/sec13.htm.

Well, that, technically, makes any shift at speed "unsafe coasting".

The 3-series is 182 in long (E90), which means, that at 60 mph, you would need to change gears within 0.17 seconds to ensure you are not "unsafely coasting" as per the description above.



Now, I get it, this probably refers to slowing down, but even at 35 mph, you would need to shift gears within 0.3 seconds! Downshift, none-the-less! Good luck, boys and girls and I want to hear some nice rev-matched, below 300 milliseconds downshifts on my way home.
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  #13  
Old 11-29-2012, 07:07 PM
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CALWATERBOY CALWATERBOY is offline
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Quote:
Originally Posted by unrealii View Post
.... your thoughts?

I notice my car's computer displays lower mpg coasting in gear than out.
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  #14  
Old 11-30-2012, 01:19 PM
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kunal_D kunal_D is online now
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Quote:
Originally Posted by CALWATERBOY View Post
I notice my car's computer displays lower mpg coasting in gear than out.
The injectors shut off when coasting in gear and do not use any fuel. Practically every car for the last 20+ years has this functionality.
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