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Old 07-15-2008, 11:38 AM   #11 (permalink)
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Neutral is almost always best. Unless you're in a situation where you're using the brakes, don't do in-gear engine braking. The momentum loss is MUCH greater than the tiny bit of fuel not consumed.

On the highway, your engine braking will actually slow you more, due to higher engine rpm combined with aerodynamic drag.

My civic idles at 0.3gph. mph / gph gives mpg, so...
50mph / 0.3 gph = 166 mpg
40 / .3 = 133 mpg
30 / .3 = 100 mpg
20 / .3 = 66.7 mpg

So you can see, even at 20mph, rolling in neutral will be improving your average.

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Old 07-15-2008, 12:01 PM   #12 (permalink)
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On the highway, the faster you go, the higher aerodynamic drag you have. From my observation, coasting in a deep long downhill, coasting in neutral will not accelerate forever. The speed will be stable in certain point. Same to coasting in gear, but the speed will be lower. The speed gets stable because of the external drag and internal drag. External drag is aerodynamic drag and rolling friction. since coasting in neutral runs faster, higher aerodynamic drag. Internal drag is engine braking force, in this case, which coasting in gear is bigger.

From my daily commuting, coasting in neutral on a particular downhill is 70 to 75mph, depends on traffic (drafting effect) and which lane (different slop angle since the road is curvy) Coasting in gear is 60mph max. on that road.

How far you can continue coasting after downhill depends on 1) the speed the car is running at the end of the hill 2) the drafting force the car has. That is why coasting in neutral goes further because of those advantages.

If, the car is coasting in gear (fuel cut) on the first half, then coasting in neutral on the second half, the car will coast similar distance as all the way coasting in neutral, but save gas on the first half of the hill. Is this strategy right?
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Old 07-15-2008, 12:15 PM   #13 (permalink)
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In one sense, that is right. In another sense...

You used fuel going up the hill. You gain that back on the downhill, MINUS the drag being overcome, which includes aero and engine braking.

I would try to crest the hill even slower, so the trip down the slope doesn't approach terminal speed. At the very least, I'd try to minimize the time spent near terminal. To the extreme, if you crest at 2mph, you've used the absolute minimum fuel required to reach the peak, and gravity will take you all the way down for free. Obviously that's not a usable solution, but the concept can be applied in moderation.
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Old 07-15-2008, 03:56 PM   #14 (permalink)
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It seems to me, like many have mentioned that while you are in gear you are actually engine breaking while you are going "too fast" for the current gearing OR you are idling if the engine is spinning fast enough to push the car forward without stalling.

The actual amount of breaking will depend on your final gear ratio, and will be very minimal in an overdrive situation. With a manual transmission, the lower gears obviously cause higher rpm indicating that inertia is turning the engine over ( so not much fuel is used).

While in neutral (or with the clutch fully pressed), the transmission and the engine are "off-line" and out of the loop. They are not either giving or getting any energy from the momentum of the car. So there is much less friction at play so the car will coast farther.

Here's a couple examples...
While coasting in 6th gear at 70 miles an hour on a flat freeway I get 65-75 MPG on my gauge. the RPM is at about 1500. When in neutral on the same stretch at the same speed my gauge is in the high 90's, and the engine RPM is about 600. The max displayed on my car is 99MPG.

At the other end of the spectrum there are a couple of sections of 25MPH road. I "idle" these sections in 6th gear. Basically, all I have to do is get to about 30MPH (so the engine does not bog down and stall), then take my foot off the accelerator. The car will idle along at 20-23 MPH while getting ~45MPG, and since there is not any big hills the engine can continue to produce enogh power to keep the car moving with out starving itself, and stalling.
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Old 07-15-2008, 09:20 PM   #15 (permalink)
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I would never have guessed mileage is better in gear. hmm
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Old 10-18-2008, 09:02 PM   #16 (permalink)
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I drive a 5 speed Toyota corolla average mpg is 38. I started coasting in nuetral with engine off and have gotten as good a 50 mpg.
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Old 10-18-2008, 09:30 PM   #17 (permalink)
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I am starting to wonder about wear on the clutch from going in and out of gear (or starting the engine back up from EOC).

Of course, my truck has 260,000 miles on the original clutch.

The truck uses about 0.55 gph in idle, so steady state neutral coasting is about 130mpg. There is a portion of the interstate that I can EOC and maintain speed. It knocks up the MPG for the trip up by about a full digit. I do like seeing the 99999 on my MPGuino though.
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Old 10-19-2008, 10:28 AM   #18 (permalink)
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So from all this, it's better to coast in gear if you are planning on slowing down (coming to a light or stop sign); and it is better to coast in neutral when you are on the open road and using a P&G technique, especially down hills. Is this correct?
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Old 10-19-2008, 12:05 PM   #19 (permalink)
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I would still disagree with that. Let me put it this way. If you are X distance from the stop sign such that you can put it in gear and downshift as you slow down to reach a stop at the stop sign, it is better to slow down in gear than it would be to coast in neutral and use the brakes. However, it would be even better to have been thinking about this further away, Y distance away when coasting in neutral you would coast just to a stop at the stop sign, and coast in neutral from there, instead of continuing to drive normally from Y to X and then using engine braking.
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Old 10-19-2008, 01:28 PM   #20 (permalink)
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So using engine braking is only better when you need to stop sooner than coasting in neutral, but coasting in neutral is still preferred.

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