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Old 11-02-2011, 08:07 PM   #1 (permalink)
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P&G question!!!

Hey guys, excuse me for my noobness. But to be honest I got a question about P&G on an automatic transmission car. While getting an awesome kick out of it, like most of you do..I noticed whenever I throw it back in drive, it slows down. I know this because it's in gear and obviously drops my speed. But I noticed if I rev match it to the speed and the gear I assume I'm going into it doesn't "jerk" into gear. It keeps the speed.

But the main question is, what hurts my fuel economy more? Letting the car slow down, or bringing the engine about <1,000 RPMs up so I can keep the speed and continue on my way?

Should I even be P&G on an auto? I do know how to rev match correctly (previous fun and very not so efficient GTI's I'vd had on the past)


BTW, this site is freaking awesome. I've been driving more "eco-like" and so far I got 1/4 tank left to go when I would be empty by now.

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Old 11-03-2011, 06:24 AM   #2 (permalink)
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I think it's pretty much a wash. While the kinetic energy that spins up the engine (when you don't rev-match) was likely generated at higher mean engine efficiency than you get while rev-matching, that energy had to go back and forth over the drive line, causing significant losses.

So go ahead and rev-match if it makes the journey smoother.
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Old 11-03-2011, 02:06 PM   #3 (permalink)
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Quote:
Originally Posted by Snatcher View Post
But I noticed if I rev match it to the speed and the gear I assume I'm going into it doesn't "jerk" into gear. It keeps the speed.
Jerkiness is not good in anything mechanical.
Rev-matching smooths out things, so it's OK, even if it means blipping the throttle a bit and using fuel.

Quote:
But the main question is, what hurts my fuel economy more? Letting the car slow down, or bringing the engine about <1,000 RPMs up so I can keep the speed and continue on my way?
Braking through the engine when you don't need to slow down, is a waste of fuel, as you'll have to bring the entire heavy car back up to speed.
That's going to cost more fuel than briefly blipping its unloaded engine to do 1000 rpm more.

Quote:
Should I even be P&G on an auto?
Do you keep the engine running during the glide, or not ?

Shutting down the engine could harm an automatic due to lack of lubing.
If it can be flat towed for extended distances, you're OK.
If not, keep the engine running.
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Old 11-03-2011, 06:41 PM   #4 (permalink)
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Well I figured as much. I meant the jerkiness when it goes back into gear from neutral. Yeah I don't turn off the car when I'm gliding. I rather not, reading up on what I should be and shouldn't be doing pointed it out.

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Old 11-03-2011, 07:32 PM   #5 (permalink)
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Quote:
Originally Posted by Snatcher View Post
Well I figured as much. I meant the jerkiness when it goes back into gear from neutral. Yeah I don't turn off the car when I'm gliding. I rather not, reading up on what I should be and shouldn't be doing pointed it out.

Thanks man!
Looking quickly online, it seems your xB can't be flat towed, so don't glide with the engine off. You'll damage the tranny. Keep the engine on and you'll be fine. To confirm this, check under dinghy towing in your owner's manual.
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Old 11-03-2011, 07:53 PM   #6 (permalink)
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I have been able to verify that it is possible to pulse and glide with an automatic transmission. There is no need to shift to neutral, or do any sort of hardware modifications to your car. I've done this myself with my truck.

The idea is, once you're at speed, try to briskly accelerate to a few miles above your desired speed, then let off the gas to allow your vehicle to slow down to a few miles below your speed. Then, accelerate to go to a few miles above your speed, and so on and so on...

If you accelerate and you find that your vehicle downshifts on you, you're using too much gas. Back off a little on the gas pedal, and try again. Also, if you have access to something like a ScanGauge, and you find that your ScanGague is telling you that your vehicle goes into open loop when you accelerate, you're using too much gas. If you use your brakes, well... that should be obvious that you just converted some gasoline into brake dust.

I've discovered this method on my daily commute to work, where I would oftentimes be caught in heavy (but still moving) traffic. I used to wonder why I'd get as much as 8% better fuel economy on days that I'd have to deal with heavy traffic, than on days where traffic was relatively light. In either case, my travelling speed was not a contributor to increased FE. Heavy traffic on my commute typically travels about 8 over the speed limit, and I normally aim for 7 over.

I first noticed this by noting total trip fuel consumption as measured on my Scangauge II at mile 25 of my commute into work. On heavy traffic days, I have observed as much as 21.2 and usually around 20.5 MPG. On light traffic days, I have observed around 19.4 to 19.6 MPG this past month. I have subsequently tested my "P&G" theory this past Sunday (a guaranteed light traffic day on my commute into work, thus no possibility of "drafting" to explain my FE gain, and no possibility of annoying somebody behind me), and got 20.9 MPG.

So, yes, it is possible to P&G with an automatic. It's just different than for a manual.
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Old 11-03-2011, 10:35 PM   #7 (permalink)
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On my 02 Insight I could coast down to 16 MPH and still get 150 MPG with the engine idling. It had an instant FE bar graph on the dash and it would read max at 150 MPG down to 16 MPH coasting in neutral with engine on.

It worked out to .11 GPH idling, while coasting, with no accessory loads including radio.
You needed to rev match the re-engagement of the transmission in the Insight.

On the 08 Altima, also with the CVT, you could coast down to 16 MPH while still maxing out the bar graph instant MPG reading at 60 MPG, which was as high as it could read.
On some of my local back roads with 35 MPH speeds I could get very close to 60 MPG average in the Altima using pulse and coast (engine on).

In the Insight it was close to 90 MPG on the same route.

I never coasted engine off in either vehicle but when coasting engine on, I divided the coasting speed by 16 and the mulitplied it by 60 (Altima) and 150 Insight to get my higher speed coasting MPG. Used wisely it could make a significant difference in overall MPG.

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Old 11-03-2011, 11:09 PM   #8 (permalink)
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Quote:
Originally Posted by t vago View Post
The idea is, once you're at speed, try to briskly accelerate to a few miles above your desired speed, then let off the gas to allow your vehicle to slow down to a few miles below your speed. Then, accelerate to go to a few miles above your speed, and so on and so on...

If you accelerate and you find that your vehicle downshifts on you, you're using too much gas. Back off a little on the gas pedal, and try again. Also, if you have access to something like a ScanGauge, and you find that your ScanGague is telling you that your vehicle goes into open loop when you accelerate, you're using too much gas. If you use your brakes, well... that should be obvious that you just converted some gasoline into brake dust.

I've discovered this method on my daily commute to work, where I would oftentimes be caught in heavy (but still moving) traffic. I used to wonder why I'd get as much as 8% better fuel economy on days that I'd have to deal with heavy traffic, than on days where traffic was relatively light. In either case, my travelling speed was not a contributor to increased FE. Heavy traffic on my commute typically travels about 8 over the speed limit, and I normally aim for 7 over.

So, yes, it is possible to P&G with an automatic. It's just different than for a manual.
Totally understand, but when doesn't the transmission stay in gear and slow down a lot quicker than when it's not in neutral? I've seen on my commute if I leave it in gear it slows down much quicker that it would in neutral. Wouldn't that help me get my car farther then have the tranny in gear to bring it back up to speed. Sometimes (obviously) when I go downhill I throw it in neutral (around 55mph) and it keeps speed real well than it would do in gear. Most of the time it actually gains a tiny bit of speed.

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Originally Posted by Old Mechanic View Post
On my 02 Insight I could coast down to 16 MPH and still get 150 MPG with the engine idling. It had an instant FE bar graph on the dash and it would read max at 150 MPG down to 16 MPH coasting in neutral with engine on.

It worked out to .11 GPH idling, while coasting, with no accessory loads including radio.
You needed to rev match the re-engagement of the transmission in the Insight.

On the 08 Altima, also with the CVT, you could coast down to 16 MPH while still maxing out the bar graph instant MPG reading at 60 MPG, which was as high as it could read.
On some of my local back roads with 35 MPH speeds I could get very close to 60 MPG average in the Altima using pulse and coast (engine on).

In the Insight it was close to 90 MPG on the same route.

I never coasted engine off in either vehicle but when coasting engine on, I divided the coasting speed by 16 and the mulitplied it by 60 (Altima) and 150 Insight to get my higher speed coasting MPG. Used wisely it could make a significant difference in overall MPG.

regards
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Nice, I only turned the engine off like 5-10 times, until I did more research and I found out I'm not supposed to be doing it. Not doing any good for the tranny.

Just curious, how's the CVT working out as in MPG. My manager has the same car, with CVT. From what I know CVT keeps the engine at a "sweet spot" for efficiency I assume. Does it give good basic gas mileage?
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Old 11-03-2011, 11:42 PM   #9 (permalink)
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Quote:
Originally Posted by Snatcher View Post
Totally understand, but when doesn't the transmission stay in gear and slow down a lot quicker than when it's not in neutral? I've seen on my commute if I leave it in gear it slows down much quicker that it would in neutral. Wouldn't that help me get my car farther then have the tranny in gear to bring it back up to speed. Sometimes (obviously) when I go downhill I throw it in neutral (around 55mph) and it keeps speed real well than it would do in gear. Most of the time it actually gains a tiny bit of speed.
You may be able to go farther if you put your transmission into neutral while coasting, than if you leave it in gear, but consider this:

Every time you shift from neutral to drive, you're inducing a tiny bit of wear on the friction component of your transmission's geartrain. This is by design and is unavoidable. Also consider that automatic transmissions were designed to be shifted from neutral into gear while the vehicle is not moving, so as to minimize that wear I just mentioned. Shifting the automatic transmission in and out of neutral, while the vehicle is moving forward at a decent speed, is a sure way to shorten the life of the transmission, because you'd be inducing a lot of unnecessary wear in the geartrain friction component of the transmission.

Finally, there is a safety aspect that you might not be considering (or that you might be discounting). If you were to need to be able to accelerate out of a situation, it's far better to just have to step on the gas, than it would be to shift the car back into gear if it were in neutral, and then step on the gas.

By comparison, the lockup torque converter (at least in my truck) is designed to unlock and lock up while the vehicle is in motion, and while a significant amount of power is being fed from the engine into the transmission. My torque converter will alternately lock-up and unlock whenever I am stepping on the gas, then letting off of the gas pedal. However, that isn't going to wear out my lockup torque converter very much, and it's certainly safer than shifting to neutral and back.

Therefore, I recommend that your transmission be left in gear. The small amount of gasoline you'll save by switching in and out of neutral, above and beyond the pulse-n-glide technique I described, isn't worth shortening the life of your transmission, nor is it worth reducing your safety margin of error.
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Old 11-04-2011, 02:19 AM   #10 (permalink)
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OP- thanks for posting this I have the same concerns and I've been battling this for a bit.

Quote:
Originally Posted by t vago View Post
You may be able to go farther if you put your transmission into neutral while coasting, than if you leave it in gear, but consider this:

Every time you shift from neutral to drive, you're inducing a tiny bit of wear on the friction component of your transmission's geartrain. This is by design and is unavoidable. Also consider that automatic transmissions were designed to be shifted from neutral into gear while the vehicle is not moving, so as to minimize that wear I just mentioned. Shifting the automatic transmission in and out of neutral, while the vehicle is moving forward at a decent speed, is a sure way to shorten the life of the transmission, because you'd be inducing a lot of unnecessary wear in the geartrain friction component of the transmission.
good point. In my 2012 Focus it's actually a maunmatic (automated manual) So I don't have the same concerns as the OP. But I agree with you here. IMO on the XB you're looking at a vehicle not designed to be shifted in and out of gear at speed. Be sure to check if you can flat tow the Scion and what speeds are recommended. If you can't flat tow it, don't shift into neutral while rolling.


Quote:
By comparison, the lockup torque converter (at least in my truck) is designed to unlock and lock up while the vehicle is in motion, and while a significant amount of power is being fed from the engine into the transmission. My torque converter will alternately lock-up and unlock whenever I am stepping on the gas, then letting off of the gas pedal. However, that isn't going to wear out my lockup torque converter very much, and it's certainly safer than shifting to neutral and back.

Therefore, I recommend that your transmission be left in gear. The small amount of gasoline you'll save by switching in and out of neutral, above and beyond the pulse-n-glide technique I described, isn't worth shortening the life of your transmission, nor is it worth reducing your safety margin of error.
Another good point here. I'm trying to realize what the shift to neutral is gaining me and I think it's actually significant. Next tank I'm going to not neutral shift at all and find out (ok in 2 tanks actually next tank is already claimed for another test phase). I'll report back. I suspect my 34mpg tank was due to no neutral shifts while I'm now getting 38mpg almost all the time.

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