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Old 04-06-2008, 01:23 PM   #1 (permalink)
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Coasting in gear vs. neutral (fuel cut feature)

So, yesterday I was messing about with my scangauge and made a breakthrough: when I coast down hill in neutral (no engine off yet for me) my car consumes gas at idle level, but if I coast in gear taking my foot all the way off the pedal, after a few seconds my gallons-per-hour readout hits 0.00, which leads me to believe this is closer to EOC than when I go into neutral down hills. My question then, is how does this actually compare to other methods to boost MPG down hills? I assume cutting the engine and going into neutral would yield a greater increase in speed down hills, and therefore higher MPG than my coasting in gear, but the SG reads 9999 MPG until I use the throttle again so I have assume this could be better than engine on coasting.

This might not be anything new to some of you, but I had no idea my car was cutting fuel when coasting in gear.

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Old 04-06-2008, 05:05 PM   #2 (permalink)
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Many cars have fuel cut-off when deccelerating, usually above a predetermined speed (i.e. below ~30mph injectors may fire). Like you said, coasting in gear will slow you down compared to staying in neutral.

Lets say you can go 1000ft coasting in neutral at 99mpg, and 500ft coasting in gear at infiniti mpg. To see which one is best, you have to average your mpg over the distance you can achieve in neutral, 1000ft. You could probably use the trip function of Scangauge to determine this. The coast in gear will force you to drive 500ft, so it may balance out.

If the trip feature doesn't work, the easiest way is to find fuel consumed over that 1000ft distance. I'm not sure if Scangauge has a fuel consumed gauge, but it should...

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Old 04-06-2008, 05:53 PM   #3 (permalink)
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Indeed, many cars have fuel cut when coasting in gear, the trick is to balance lost speed with reduced fuel use.
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Old 04-06-2008, 08:26 PM   #4 (permalink)
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My car does better by far coasting in neutral. Engine braking slows the car down too fast.
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Old 04-06-2008, 09:28 PM   #5 (permalink)
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For me its coasting in gear (automatic) so I dont have to worry about what happens in neutral at 50mph with my slushbox.
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Old 04-06-2008, 09:39 PM   #6 (permalink)
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I shift in and out of gear all the time on all my automatics. No problems in 20 years.
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Old 04-06-2008, 09:52 PM   #7 (permalink)
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Quote:
Originally Posted by SVOboy View Post
Indeed, many cars have fuel cut when coasting in gear, the trick is to balance lost speed with reduced fuel use.
+1

In most freeway situations, it pays off to coast down hill in neutral. Sometimes I need to control my speed so I keep it in gear. And other times I know i need to come to a complete stop so I coast in gear. Its all dependent on the situation.
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Old 04-06-2008, 09:54 PM   #8 (permalink)
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the rabbit diesel does not cut fuel off on decel much to my amazment. it goes to idle fuel. so i put a button on the shift lever to cut power to the injection pump. during decel in a lower gear, i can actually feel it slow down when i hit the button. but for max mileage it is far better in neutral. because i coast three times as far.
My gasoline truck does cut fuel over a certain RPM, but it take too long for it to cut and it comes back on way too early. so neutral is far better there also.

Next i am going to cut the VSS signal during neutral coast on my windstar because ECU tracks the vehicle speed with the engine even in neutral, dumb computer.
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Old 04-11-2008, 08:45 AM   #9 (permalink)
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Here's the phenomenon in action.

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Old 04-22-2008, 12:53 PM   #10 (permalink)
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My manual varies with the slope. Sometimes it's best to shift to neutral, sometimes it's best to leave it in gear. I listen to the engine and watch the rpms.

I don't think one method is going to work for all cars and conditions, so you'll have to experiment.

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