MPG? (Hypermiling, Scanguage question)

forestacademy · 07-28-2009, 01:09 PM

I have a question for folks with a scanguage or other device on their car.

In past manual cars I had I would put the car into nutural to coast to stop signs. When stitting at lights I would also put it into N while I wait for a green. With the smart I have been doing the same thing.

My question is, when coasting on level ground from about 45 mph to a stop, what is the mpg difference with the car in N verses D?

I am also wondering if their is a difference between N with a light break or D With a heavy break at the stoplights.

Lets keep this out of the discussion of is it safe to roll in N if we could

.

I drive on lonely country roads and can coast in some instances for 1/2 mile or so and still have to break at the stops and no one else on the road

Dave777 · 07-28-2009, 01:43 PM

If you're coasting in N the engine is idling, burning fuel.

If you're coasting in D the engine fuel is cutoff completely until the speed drops to a point where the engine will quit unless fuel is added to keep it running.

forestacademy · 07-28-2009, 01:54 PM

I had heard that once before.

But engine friction engaged (D) seems to slow me down more than (N). And the rpm's seem to be lower in N, Why would the fuel cutoff to the point where then engine will quit be any different in N than D.
That's why I want someone with a Guage to tell me what comes out better.

sato7 · 07-28-2009, 02:04 PM

what position you have the tranny.. D or N has nothing to do with the RPMs of the engine. when the acc pedal is not pressed the engine keeps some of the RPMs that the tranny is back feeding it. Now I'm not sure what fuel consumption this has but if the Tranny is disengaged.. N than the engine quickly slows down to its idle speed, thus using less fuel due to the faster process. an engine will always use up more fuel given the same RPMs but with load vs. no load.

John_H · 07-28-2009, 02:41 PM

The fuel consumption measurements won't communicate the difference between engine braking with zero fuel and coasting in neutral in a sensible fashion because the loss of momentum (engine braking) isn't considered.

NCC1701 · 07-28-2009, 03:31 PM

John, I haven't checked to compare the two conditions yet (coasting in N v.s. in gear) but, the SGII can show GPH. I know it reflects increased consumption of A/C v.s. no A/C while sitting in P. Wouldn't it give a valid comparison for coasting in or out of gear as well? Or, is the coasting in neutral GPH figure meaningless (if the engine is using no gasoline it should read 0 GPH).

In any case, as forest noted, the car coasts much more freely in N. You can cover a good deal of ground slowing from 55 down to 25-30 MPH in neutral. Even if the engine is running at idle that's a lot lower consumption that you'd see under power at those speeds. And, you cover more ground than you would in gear.

I'm gonna check the gauge's GPH reading for 0 MPH, in gear v.s. neutral.

forestacademy · 07-28-2009, 03:34 PM

Don't melt while doing it NCC1701

NCC1701 · 07-28-2009, 03:39 PM

Quote:

Originally Posted by forestacademy

Don't melt while doing it NCC1701

Not to worry, A/C will be in use!

If I keep it on it shouldn't affect the readings relative to each other.

**rfernatt** · 07-28-2009, 04:07 PM

I see 9999MPG on the SG when coasting in gear, but coasting in N uses some fuel so you only get 100 MPG or so depending on coast speed...

I coast in gear to stop signs and turns a lot, so you see the mileage start climbing when you take your foot off the gas and it maxes out at 9999. Idling in N uses like .3 GPH or some small amount depending on A/C, etc.

NCC1701 · 07-29-2009, 05:12 PM

I checked the GPH reading on SGII today while coasting. Coasting in gear the reading drops to 0.0 GPH after a few seconds. Same coast, went to neutral and it went to around 0.44 GPH.

So, if you coast for 2.27 hours in neutral, you'll use one gallon ... and since you've obviously found a way to defeat all friction, you'll get 136 MPG at 60 MPH.