Trick for P&G On an automatic.. Hint, you need Cruise Control - Fuelly Forums

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Old 06-24-2008, 10:56 AM   #1
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Trick for P&G On an automatic.. Hint, you need Cruise Control

I think I discovered the answer for those who would like to do pulse and glide on an automatic. While I have not tried this on any other cars thus far, I believe it will certainly work with Toyotas equipped with cruise control. I have an automatic, actually at the moment, all the cars I have are automatics. One problem is, if you wanted to do Pulse and Glide, you have to shift into neutral in order to coast which is fine but to shift BACK into drive while you're still moving WILL damage the transmission, for an automatic at least. Now if you tried to do the Pulse and Glide with out turning off the engine and just letting off the gas, you'll end up with engine braking which is BAD because that prevents you from GLIDING.

I noticed something when I have cruise control enabled and I catch a slope. For me, at 55mph the tach reads around 1500rpm, that is what it takes in order for it to maintain speed on the highway due to gearing of the transmission, fair enough. So I have cruise control set at 55mph and I catch a slope, now the car is accelerating to 60mph, but if I watch the tach, something happens. Instead of the RPMS increasing in line with the higher speeds which would lead to engine braking and the quick slow down to 55mph, something else happens. Since an automatic can engage and disengage the transmission at any time with degrees of engagement in between, what I'm about to tell you make makes perfect sense. So what I see are the RPMs drop to 1200, then 1000 and I think at one point 800. This means that the transmission goes into neutral when the car accelerates faster than the set speed of the cruise control and when the throttle isn't used, i.e. when coasting down a hill. I'm sure it does this in order to prolong your coast and for you to enjoy the extra speed you're picking up by coasting down the hill.

This is great news, when I'm going faster than my set cruise controlled speed, the transmission goes into neutral in order to not cause engine braking which means it won't hurt my coasting at the higher speed. Ok so how can this help a hypermiler who wants to glide for long as possible while at the same time not damage their automatic transmission? Well.... since noticing this while I'm driving on the highway, before I get off I disengage the cruise control and try to coast for as long as I can. Unfortunately if I disengage it too quickly, I'll slow down so quickly and will end up having to speed up temporarily. The reason I'd slow down faster than I'd want to is because of the engine braking that normally occurs when your foot is off the throttle and cruise control isn't enabled.

If you look at the cruise control knob on a Toyota, you'll see three arrows, one pointing up, one down and one outward. The up means "accelerate" and the down means "decelerate" and outward meaning cancel. One tip people have mentioned is using cruise control to accelerate at a very moderate pace in order to save fuel. Well cruise control can come in handy for another thing, for when you want to GLIDE. Gliding is a problem on an automatic unless you have no respect for your transmission or all your routes are down hill on at least 5% grade because of engine braking.

So when I discovered that the transmission disengages when the car goes faster than the set speed of cruise control so that you will coast while at the higher speeds, I figured I'd try something else. Instead of canceling the cruise control right before I got off the highway in order to coast, I instead held the cruise control knob downward in order to keep telling the cruise control to "decelerate", meaning to reduce my set cruise control speed. What this did was effectively do the same thing as when I had the cruise control set to 55mph and the car accelerates to 60mph due to other forces besides the throttle such as being on an incline. So by telling the cruise control to decelerate, the transmission basically goes into neutral and I was then able to glide a much farther distance since there was no engine braking.

Summary: Pulse and Glide doesn't work with an automatic unless you have no respect for your drivetrain. You can shift out of gear while moving but to shift back on an automatic while moving WILL destroy your transmission. You can't easily glide on an automatic because you will have some sort of engine braking somewhere except in one case. If you're going 55mph, have cruise control enabled and then hold "decelerate" on the cruise control continuously, the car will not use engine braking. It will not use engine braking because in this specific scenario the transmission will self disengage and effectively be in neutral, therefore permitting an actual glide.

I have found this only to be true on one car which is a Lexus, but the cruise control functions similarly on all toyotas. I plan to test this on my other cars when given the chance and I believe they will all peform the same due to them acting the same in similar circumstances. I do not know if this will work for you but there is no harm in trying. I would not be surprised if this was limited to only new toyota based cars and that were made in the last 5 years.
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Old 06-24-2008, 11:01 AM   #2
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So if you put your transmission in neutral it will explode, but if your cruise control does it it won't? oooookayyyyyy...
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Old 06-24-2008, 11:19 AM   #3
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I can't imagine why shifting from N to D will cause damage. Has it happened to you?

I've been P&Ging in my GMC for a while now (1 month, ~1200 miles) and had no problem, even though the transmission I have is supposedly weak. I've also used N randomly (mainly out of boredom) since I began driving 12 years ago. I've never had a transmission problem.

Also, I seriously doubt that your computer shifts the transmission into neutral when the cruise control is set. Try the following experiment:
- First, try your recommended procedure, and note the exact RPM it eventually settles to at a given speed
- Then, try it without cruise control, just take your foot off the gas, and again note the RPM it settles to at the same speed

I doubt they will differ.

- Finally, try it in N (if you're not afraid to). See if it settles to even lower RPM.

Automatic transmissions are equipped with a torque converter, which doesn't mechanically connect the engine to the transmission. Instead, it connects them (this explanation is a bit simplified but gets the point across) using two opposed turbines pushing transmission fluid at eachother. This means that, at any given speed, the engine can go a lot faster or slower than if it was connected with a clutch from a manual transmission. It also means that you can be in gear at a complete stop and the engine doesn't stall. It also means that you can be in gear, coasting down a hill, and the engine can settle down to near idle speed without the transmission having to disengage the gear.
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Old 06-24-2008, 11:21 AM   #4
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Originally Posted by RoadWarrior View Post
So if you put your transmission in neutral it will explode, but if your cruise control does it it won't? oooookayyyyyy...
The transmission shifts in and out of neutral all the time, especially when shifting gears, the difference is, it's in a controlled manner and it's by design. It's not part of the design for you to manually shift it into neutral and then shift it back into gear while moving. When was the last time you moved a lever in order to get the transmission to lock up the torque converter? You don't, it does it for you when the conditions are right. It's far different to move the shifting in and out of neutral than for the transmission to do it for you.


Here is an example: You're cruising at 25MPH in 5th gear, you floor it in your automatic, what happens? The transmission smoothly shifts into a lower gear, and how low depends on a lot of factors but may go as low as 1st gear. Now if you read the manual, it says not to press on the accelerator when shifting into a lower gear. But isn't that the same thing? Well it is and it isn't, when the manual speaks of not accelerating while shifting into a lower gear, it's because the driver took the shifter and put it into a lower gear while accelerating, BAD IDEA. But when you simply press on the throttle and have the transmission shift for you, then it's ok. This is less of a problem on newer automatics since in my experience will prevent you from shifting into too low of a gear by giving an audible alert and staying in gear. On an older automatic, there is no such protection and it WILL let you shift into a lower gear even if that means redlining it. There are a lot more protections now than there were 10 years ago.

If you've ever accelerated while moving the shifter into a lower gear (not saying you should, it's a BAD IDEA!) you'll hear some bad sounds and it'll feel very wrong, because it is. But when I floored it in my automatic, it downshifted AS I was accelerating yet it doesn't sound wrong or funny by any measure and that's because it's in a controlled manner while shifting into neutral and then back into Drive is NOT.
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Old 06-24-2008, 11:22 AM   #5
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Originally Posted by theholycow View Post
I can't imagine why shifting from N to D will cause damage. Has it happened to you?

I've been P&Ging in my GMC for a while now (1 month, ~1200 miles) and had no problem, even though the transmission I have is supposedly weak. I've also used N randomly (mainly out of boredom) since I began driving 12 years ago. I've never had a transmission problem.

Also, I seriously doubt that your computer shifts the transmission into neutral when the cruise control is set. Try the following experiment:
- First, try your recommended procedure, and note the exact RPM it eventually settles to at a given speed
- Then, try it without cruise control, just take your foot off the gas, and again note the RPM it settles to at the same speed

I doubt they will differ.

- Finally, try it in N (if you're not afraid to). See if it settles to even lower RPM.

Automatic transmissions are equipped with a torque converter, which doesn't mechanically connect the engine to the transmission. Instead, it connects them (this explanation is a bit simplified but gets the point across) using two opposed turbines pushing transmission fluid at eachother. This means that, at any given speed, the engine can go a lot faster or slower than if it was connected with a clutch from a manual transmission. It also means that you can be in gear at a complete stop and the engine doesn't stall. It also means that you can be in gear, coasting down a hill, and the engine can settle down to near idle speed without the transmission having to disengage the gear.
I don't think you read my post, read it again.
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Old 06-24-2008, 12:11 PM   #6
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I think it means that you don't need to use the throttle pedal to rev match when using the lower gear for engine braking on hills.

Pop quiz hotshot, to bring out your inner auto tranny expertise, what's better hard firm shifts or smooth as butter shifts you can barely feel?
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Old 06-24-2008, 12:19 PM   #7
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Old 06-24-2008, 12:21 PM   #8
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When was the last time you moved a lever in order to get the transmission to lock up the torque converter?
Last Friday, although it was a button instead of a lever, and it's a mod that I put in.

Toyota automatics generally disengage the TCC but remain in gear at closed throttle. The lower engine speed that you're seeing is the engine being backdriven through the torque converter, which is dissipating kinetic energy in the TC fluid and engine. If you compare high speed coasting distances (e.g. 60-40 MPH) at "decel" and in neutral you should find that the coastdown distances in neutral are longer.

Even though my car isn't rated for flat towing, I've been EOCing for at least 15,000 miles without any problems so far (knock on wood).
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Old 06-24-2008, 02:32 PM   #9
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Originally Posted by Bruce View Post
Last Friday, although it was a button instead of a lever, and it's a mod that I put in.

Toyota automatics generally disengage the TCC but remain in gear at closed throttle. The lower engine speed that you're seeing is the engine being backdriven through the torque converter, which is dissipating kinetic energy in the TC fluid and engine. If you compare high speed coasting distances (e.g. 60-40 MPH) at "decel" and in neutral you should find that the coastdown distances in neutral are longer.

Even though my car isn't rated for flat towing, I've been EOCing for at least 15,000 miles without any problems so far (knock on wood).
15,000 miles is nothing, just like the above poster who said 1500 miles. Anyhow the point was, using the cruise control to keep the car in neutral with out actually shifting in neutral would be better for the transmission than actually shifting it for the reasons I outlined above. And to further add that 15,000 miles is nothing, I know of a guy who drove around 100,000 with out ever changing his transmission oil before something went wrong. This was on a very old GMC Sierra so it wasn't one of those vehicles that are meant to go 100,000 miles between transmission fluid changes.
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Old 06-24-2008, 03:19 PM   #10
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I going to give this a try on the "Do Not Flat Tow" iAWD auto SX4. There's not many hypermiling techniques that I can do with this car, but this might work. Thanks for the tip!
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