Engine Braking vs. Neutral

[jefferios]

So whats better for our fuel economy.

1. Going 35mph down a hill and coasting in 3rd or 4th gear (Keeping a constant speed)

2. Going 35mph down a hill and manually going up to 5th gear (Accelerating slowly)

3. Going 35mph down a hill and putting the car into neutral.

[forestacademy]

When I Asked a similar question before, the scanguage folks said the engine engages will cut the fuel while still running the alternator, as opposed to riding in N will require the engine to keep fuel flowing to keep the thing going and the alternator charging.

I had asked the question in regards to slowing down to a stop and there the options are not as clear because the car would stop sooner keeping it in gear, but going down hill is a different issue. Take advantage of the gravity.

[rfernatt]

I say #1. Coasting downhill in gear should shut off the fuel (or be very close to completely off). I do this every morning for 1.5 miles. Of course, every evening, I have to climb back up...

[gurps]

I'm not so convinced of a 'fuel shutoff' when engine braking. I'd like to someday use an OBD tool to monitor fuel flow and find out for sure. I suspect that for emisions purpose that the fuel is NOT cut off, but would like to prove it either way.
I currently do both techniques( in gear and N), in my quest to get better mileage.

I find that the thing I can do most to help is anticipate: coming up to the crest of a hill, knowing that there is a stop sign 2 blocks past the other side, seeing a light turn up ahead in the distance - that kind of thing.

[jimvw57]

In some states I believe coasting downhill in neutral is illegal, at least that is what I have herd. I don't have any way to prove or disprove it. But it would seem that the computer would be confused with an input saying it was traveling at 35 MPH and the motor was rotating too slow..

[stevet47]

Originally Posted by gurps

I'm not so convinced of a 'fuel shutoff' when engine braking. I'd like to someday use an OBD tool to monitor fuel flow and find out for sure. I suspect that for emisions purpose that the fuel is NOT cut off, but would like to prove it either way.
I currently do both techniques( in gear and N), in my quest to get better mileage.

I find that the thing I can do most to help is anticipate: coming up to the crest of a hill, knowing that there is a stop sign 2 blocks past the other side, seeing a light turn up ahead in the distance - that kind of thing.

It is called DFCO (deceleration fuel cut off) cars have done it for years. It is legit.
I dont think there is one best answer, it depends on the hill, what comes after the hill etc.
If there is a stop sign at the bottom, then downshift and go into DFCO.
If there is an uphill at the bottom, go into neutral and pick up as much speed as you can to coast as far up the next hill as possible.

[chieftmc]

Originally Posted by gurps

I'm not so convinced of a 'fuel shutoff' when engine braking..

It CAN be seen on a SGII

[rfernatt]

Originally Posted by gurps

I find that the thing I can do most to help is anticipate: coming up to the crest of a hill, knowing that there is a stop sign 2 blocks past the other side, seeing a light turn up ahead in the distance - that kind of thing.

I definitely do this. If you know your route and monitor traffic conditions, you can do some good coasting in gear. I avoid shifting into N, but I will shift into 5th on downhill sections to limit engine braking and pick up speed for the next uphill. I will coast from 5th to stop at a few places where I can do it and not anger anyone.

[jimvw57]

This is my theory: When you let off on the gas, the engine slows down, and the transmission helps slow down the car. With the car in neutral, the engine is influenced by the computer which can tll you are still moving so it keeps the idle speed higher. If for some reason you had to put the car back in gear, the jolt between the engine slow speed and the wheels turning could be a bad thing for that poor little transmission to handle.

Just my thoughts on the matter. I have also been told that coasting in neutral is illegal in some states, I don't know which ones but I was told that.

[jefferios]

Okay. Well in Duluth there is no such thing as a drive where you are going up and down. Its one of the other. If your going downhill the end is Lake Superior...so a stop sign is going to appear eventually.

Neutral makes sense if you have another hill to climb after going down. But in my case I have steep drops in a short distance most of the time.

And sometimes the engine will rev to 3500rpm if I stay in 3rd gear versus going into 5th and picking up speed on the same hill.

So is it better to go down a 30mph limit hill in 3rd gear (higher rpms) versus 5th if your eventually going to have to stop after a few miles.