Coasting: In gear vs. Neutral...

[7milesout]

I've been meaning to post this for some time. This discussion is primarily for us 5MT folks, but I suppose it has some relevancy to the CVT folks also. Just easier for a 5MT driver.

As of late, I have noticed that my mpg has taken a fairly sharp uptick when, instead of just letting off the gas pedal way early and coasting it down in-gear, that I push in the clutch and pop it into Neutral. I try to put it into Neutral even before I'd let off the gas pedal early to coast down in-gear.

Without getting into how bezerk all the tailgaters go, who are apparently eager to minimize their mpg & brake life, I am surprised by this sharp uptick.

I have been under the assumptition that when I'm rolling along at say 2,750 rpm in 5th gear, and I let off the gas pedal, that the engine goes to a state of zero gasoline injection either at the instant I let off, or sometime soon after I let off. But now I think this is wrong, based on my mpg improvement from coasting in Neutral. I would have bet coasting in-gear would have resulted in the higher mpg results, I would have lost that bet.

As I understand, this zero fuel input when letting off the gas in gear IS TRUE for my diesel truck. My diesel (and maybe all diesels) has no throttle valve in the intake. So there's less engine braking based on vacuum with off-throttle "coasting" in my diesel. Any little input of fuel in gear while off-throttle coasting and a diesel might "coast" a hundred miles (I exaggerate for effect) due to having no throttle valve.

Of course our 3-tapper has a throttle valve. And now (after realizing this uptick) I suppose it must be injecting some really small amounts of fuel even when off throttle coasting down in gear, or I wouldn't be seeing such an uptick in mpg. I'll add that what I'm doing is sort of like what hyper-milers call pulse and glide. I'm doing the "glide" part of that, and only pulse IF whatever impediment way out in front clears up. I'm not going to be pulsing and gliding on the regular, that's annoying (for me) as all hell, I can't imagine how annoying it would be for traffic behind. But when a known or dynamic slow down is coming, I hit Neutral as quick as I can and let it slowly coast down.

Can anyone who is familiar explain why I (we) get such an uptick from this? Was I wrong about the coasting down IN GEAR with zero fuel input? I realize that when I pop into Neutral, it is injecting enough fuel to idle the engine. But at ~700 rpm and no throttle input, it is consuming only a tiny amount of fuel. I'm liking this uptick. I'd say I'm able to increase my around town mpg by 3 or 4 mpg. I can hit 50 - 52 mpg on the display regularly, which works out to about 47 mpg (versus 43 mpg). I'm pleasantly surprised with this.

And just wanted to know if you other 5MT drivers have already been doing this, stumbled upon it like me, have never tried it, and/or have a more thorough explanation as to the difference in fuel consumption between in-gear coasting and Neutral coasting.

All input and thread digression welcome! Thanks!


7milesout

[jonesboro]

I haven't done a real measurement yet, and I'm not doing anything special to maximize MPG at this moment, but the dash gauge claims I get about 42 MPG all around and I do coast in neutral when slowing down. I just thought that's how you were supposed to do it lol.

I did just teach myself to drive manual a little over a month ago so I'm no expert either way.

[BigMW]

I have a 65 km, one way trip to work. There are a few long hills along the way. The highway is 80 km/h speed limit, with one shorter section at 90 km/h limit. I put almost 12,000 km so far, mostly on that highway. Recently I added a scanguage 3, and I have been monitoring it for the last 1000 km or so. Scanguage is a lot more accurate for monitoring fuel efficiency than the dash display - it is pretty much dead on as compared to calculated efficiency.

So in my limited experience with scangauge 3, the instantaneous fuel efficiency shows "0" when coasting downhill in gear. It shows 1 or 2 L/100km when coasting in neutral. However, when in gear, car slows down quicker after reaching flat, and requires earlier accelerator input to climb the next hill. If I do the same in neutral, it coasts longer, and I don't have to accelerate as early up the next hill.

So I'm pretty convinced that is the reason for better fuel efficiency - coasting down in neutral costs a little gas, but it is slightly more efficient on the next uphill.

Edit: when coasting down hill in neutral, the scanguage shows less than 1 L/100 km

By the way, I only do this when there is no other traffic - there is barely any traffic ever there

[Top_Fuel]

It's been a while since anyone has mentioned this...

DFCO explained measured and how to utilize it


This quote from Loren seems to back up what you are observing...

What I've found with the Mirage is that it does NOT respond to DFCO coasts when doing pulse & glide anywhere near as well as the Yaris did. I'm not sure why. But, the Mirage does way better doing neutral coasts. (use DFCO when you actually intend to decelerate, of course)

Here is the link to his original post.

[7milesout]

I do both if I'm going to need to use the brakes. I am trying to create the habit to hit coast even earlier, and then still "early" I will rev match it into 4th, 3rd, or 2nd gear depending on the conditions. I figure Neutral coasting for a while then DFCO for a while not only saves gas, but will save brakes.

And so, I guess these responses pretty much answers most of my questions. I guess my original thinking was correct. What I was doing was DFCO. I heard that term years back, but didn't bother to retain it.

I also did consider the fact that due to starting coasting even earlier is at least a large contribution to the bump up I'm seeing in mpg. Last night I had to cruise over to the next (close) town over. Mostly down a 2 lane, 45 mph country road with hills. Then in town for a couple miles. After leaving work, going there and once pulling into my neighborhood, my mpg display was reading 60.X mpg. Which equate to about 56 mpg. That was the first time I've had the display read 60 mpg. And if any one had been following me (and there we a couple cars doing so for parts of this trip), I'm sure they wouldn't have even noticed any abnormality from my driving.

This is quite an outstanding drivetrain.

[coop8070]

Using my torque pro app I saw that the ECU will always maintain the proper air/fuel ratio at all times. So yes, coasting down hill with the cvt in drive uses more fuel than in neutral because of the higher rpm. I think this is done to insure the catalytic converter stays hot for emissions.

I have actually been thinking about setting up a relay that turns off power to the injectors under those conditions to save fuel.

My Jetta TDI cuts fuel on hills and it does save fuel and brake pads.

[Basic]

It's been a while since anyone has mentioned this...

DFCO explained measured and how to utilize it


This quote from Loren seems to back up what you are observing...

Wait til you see the trick Garage-Door-Opener-Button kill switch Loren installed to go even further with Engine Off Neutral Coasts... its so cool.