OK to coast downhill in Neutral?

[RICHARD GILLEN]

I Shifted Into N When I Was Driving A Stick Shift Down Hills And Improved My Mpg From 21 To 27 Mpg. But I Need To Read More On The Smart Car Before I Make This Move.

An As For Unicycle I Have A Quote For You.

"when I Use A Word, Humpty Dumpty Said In Rather A Scornful Tone, It Means Just What I Choose It To Mean Neither More Nor Less.

The Question Is Said Alice Wheather You Can Make Words Mean So Many Different Things.

The Question Is Said Humpty Dumpty Which Is To Be Master That's All"

From Through The Looking Glass.

[saskdiesel]

Remember, when you are coasting in gear the injectors cease to fire. At least thats how it is with my other cars.
So all you gain is a bit of distance as you are using fuel at idle.

[MinnySmart]

Originally Posted by unicycle

I have a ScanGauge, and the change is apparent instantly. When I drive downhill, the mileage reads about 45-55 mpg. In neutral, you'd expect the instant mileage to read 9999, but actually it's usually around 130-260 mpg (usually the latter). Still, 256 mpg is a lot better than 56 mpg any day ** even for 40 seconds.

Hi Unicycle. I'm glad you posed this information of the scangauge... it got me thinking... Isn't 9,999 MPG almost 40 times better than just 256 MPG?

(This is not based in fact, but my take is this...)

If the transmission senses coasting, it cuts the fuel supply to 0. Therefore, going any distance using 0 gallons is an infinite number, displayed as 9999 which is the largest number the scangauge can handle.

If you put the transmission into neutral, it can't help tell the fuel to shut off, so the fuel runs enough to idle the car. So you have whatever distance divided by some small amount of fuel (instead of none at all) giving you 256 MPG.

However, there's more to it. If you leave the car in gear and completely let your foot off the pedal, it will actually start engine breaking, which works against your momentum. If you leave your foot on the pedal, but lift off just enough to tell the engine "I don't want to accelerate, and I don't want to decelerate, I just want to coast" and you'll do the best possible (and see that 9999 on the scangauge).

Therefore, it's my opinion that it's best to leave the transmission in gear. Even if the hill is long and steep enough to help you gain speed on the way down... better to have the system tell the fuel to cut off on the way down that hill than to use fuel just to idle the car on the way down.

[madbunny]

I still want my manual compression release so I can leave it in gear and have the rolling resistance of neutral. I mean come on, it can't be that hard of a DIY mod, can it ;)

[shack01]

Maybe I'm confused. (Happens a lot) But if "the System" tells the fuel to cut off, the engine will quit. I think we are talking about coasting with the tranny in Neutral and the engine in Idle. The engine will use enough fuel to keep running and that's it. My suspicion is that with the car in gear down the hill, more fuel will be consumed as RPMs will be well above idle.

It does sound like there are two different scan gauge results using the supposed same entry parameters. Curious.

tm

P.S. I tend to coast a lot in neutral just around town.

[unicycle]

As I noted earlier, I'm hedging towards NOT neutralizing* the car downhill after our little discussions here; mainly this is because we don't have a definitive answer on the subject, and I don't want to be the one to shed light on the matter by reporting back from a trip to the Smart Service Center.

Anyhoo, to clarify the ScanGauge thing:

1. Driving down a soft slope (say, 5% grade), the car will slowly decelerate if I take my foot off the gas, but until I put it back on (even just a bit), the SG will read 9999 mpg; essentially this is like saying I'm getting infinite mileage because I'm basically using no fuel and am still moving.

2. If I put the car into N, my readout on the SG is some number over 100 mpg and often over 200 (256 was an actual example). This suggests that a modicum of fuel is still serving the engine ** enough to idle ** but not a lot. The car will still slow down (there aren't steep enough hills where I drive), but the decel is NOT as pronounced as when it's in D over the same hill.

I hope this clarifies thangs*.

* haha. This is a joke. Now laugh.

[artificial]

84544

[Karl Roth]

Originally Posted by hampyoung

Lets assume for the discussion that the car doesn't even close to speeds where you would need to brake - because that seems to be a concern for some folks here.
Its a wide open stretch, little traffic, clear view, no tailgaters **- plenty of time to "DRIVE" your car where you want to go, whether you use the engine or momentum. Heck, if we can't handle a car when its coasting (ie: approaching any stop sign/light or traffic), what would make it easier/better/safer to handle when it under direct engine power?
I'm simply trying to learn whether using neutral OK for the engine ** not whether its an appropriate driving technique.
SO, the question is still this: is it OK for the transmission and any other moving parts to be free wheeling (similar to towing the car, yes?) when an appropriate hill for coasting in neutral presents itself.

Hamp

some automatics without a rear pump will starve lubricant to the trans in neutral. There is nothing that I can see on the smart that would be bamaged by the practice. Any smart techs know of any problems with it?
karl

[SubSlr88]

Originally Posted by hampyoung

So I still do not have an answer as to WHY it could be bad to coast in neutral down a hill. Anybody have an explanation on the technical reasons?

Well, in many states it's illegal to coast! In Colorado it will net you a ticket and a pretty hefty fine - if caught. And if you run off the road, crash or cause a pileup, it's gonna be a lot cheaper if you DON'T coast. Realistically, how much gas can you really save?

[rawlus®]

if you were coasting at 40mph down the hill in N, how would one shift back into a high drive gear when necessary?