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Discussion Starter · #1 ·
Hi Gang,

Jumped into my wife's 97 GMC Yukon yesterday and was flat out amazed at how much / how easy the Yukon will coast versus my 2008 3/4 ton Dodge automatic. I mean I was on her brakes all the time as it would coast and coast and coast. There did not seem to be an engine braking friction from the drive train. Made me feel like I drive around with my emergency brake on in the Dodge......(engine brake is off)

Anyway, the dodge has 65 pounds in front, 60 in back on stock goodrich all terrain tires. I've got the auto with 3.73 rear end. 5K miles on the Dodge, 110K on the Jimmy.

This has got to affect the fuel mileage if my truck feels like it is dragging an anchor. Just surprised me it felt like night and day.

Any ideas?
 

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IDK... I can push in the clutch and coast a loooooooong way.
 

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Well your dodge is probably quite a bit heavier than the yukon
 

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Thats because the Dodge is built that way. It uses the compression for braking. Also thats how the new 6 speed autos are so the engine brake can do its job. Our 03 Yukon will coast for miles too.
 

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Discussion Starter · #5 ·
Thats because the Dodge is built that way. It uses the compression for braking. Also thats how the new 6 speed autos are so the engine brake can do its job. Our 03 Yukon will coast for miles too.
OK - glad you find there is a difference too; with all of the sophistication of the new automatics, why don't they design a "fuel efficient" setting like the special "tow / haul" setting where the auto would allow coasting without engine braking or with reduced drag to maximize fuel mileage? If I'm on the open highway, why should I have the tranny always producing drag when I let off the accelerator?

RR
 

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River_Runner I agree!!
I noticed the same thing as soon as I bought mine.
Actually if you play around in around town driving, putting it in nuetral when you can coast up to a light down hills and such you will see the overhead start picking up milage! Some one needs to make a tourque convertor for our trucks that unlocks when there is no load on it. :$:
Ill buy one!
 

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Discussion Starter · #7 ·
River_Runner I agree!!
I noticed the same thing as soon as I bought mine.
Actually if you play around in around town driving, putting it in nuetral when you can coast up to a light down hills and such you will see the overhead start picking up milage! Some one needs to make a tourque convertor for our trucks that unlocks when there is no load on it. :$:
Ill buy one!
This is kind of crazy from a fuel efficiency standpoint.

Can we shift the transmission into N without bad things happening to the tranny? Does the tranny and associated running gear still get lubricated if the transmission is in N? I know some systems where this will cause problems.

RR
 

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Hi Gang,

Jumped into my wife's 97 GMC Yukon yesterday and was flat out amazed at how much / how easy the Yukon will coast versus my 2008 3/4 ton Dodge automatic. I mean I was on her brakes all the time as it would coast and coast and coast. There did not seem to be an engine braking friction from the drive train. Made me feel like I drive around with my emergency brake on in the Dodge......(engine brake is off)

Anyway, the dodge has 65 pounds in front, 60 in back on stock goodrich all terrain tires. I've got the auto with 3.73 rear end. 5K miles on the Dodge, 110K on the Jimmy.

This has got to affect the fuel mileage if my truck feels like it is dragging an anchor. Just surprised me it felt like night and day.

Any ideas?

One thing yours is an auto the other is its not broke in yet. 5000 miles is not going to break in that quick.:thumbsup
 

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not only the transmission but a charateristic of how diesel in injected, and that if you are not on the pedal then you are not injecting fuel which causes effectively negative hp.

all rpm in a diesel motor is based on the amount of fuel that is fed to the engine.
 

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I have felt the same. But I also know that it is a heavy duty truck so all the parts and bearings are heavier and take longer to break-in. When I want to coast I put the truck in neutral and it does do much better that way, but still drags a bit.
 

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Discussion Starter · #11 ·
not only the transmission but a charateristic of how diesel in injected, and that if you are not on the pedal then you are not injecting fuel which causes effectively negative hp.

all rpm in a diesel motor is based on the amount of fuel that is fed to the engine.

DRFTR -

If I understand you correctly, you propose I try to keep the accelerator depressed very slightly even if I am in a coasting situation just to keep the motor producing a positive push versus a negative drag (which occurs when the accelerator is not depressed). I'm new to diesels, so if this is the correct driving technique, I'll work on mastering it. Please shoot back another note here to verify.

Thanks,

RR
 

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There is another thread on here titled "Hypermiling" that is having a similar conversation.

I've been experimenting with this quite a bit the last couple of days. If the engine brake is off then taking your foot off the go pedal does allow it to roll along pretty well. If the EB is on then when your foot is off the pedal then it makes a cool noise and slows down. The effectiveness of the slow down depends greatly on whether the EGR is on and whether you are in tow/haul, and perhaps other stuff like regen etc. In any case there is decent roll in D with your foot off the gas...in fact less slowing than a gasser.

But the RPM is still up so that big engine is being turned over so the question is how tough on the tranny is it to drop it into N as you coast down a long incline or approach a stop light? Is it worth the fuel savings and perhaps engine wear or is it costly because the tranny is not being properly lubed?

I really would like to know the best procedure for rolling without power as I often drop into N. I had a 1994 Subruban that went almost 200K miles on the tranny and the mechanic couldn't believe it...said he never saw one go over 120 before. But my wifes minivan needed a new tranny under warrantee at about 10,000 miles...caused by this?
 

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whether you have the truck in N or in D and your foot is off of the accelerator. you burning the same amount of fuel. When you are at speed and you begin to coast(foot off of the accelerator) its your tranny keeping the rpms up(TC Lock up), not the engine. so putting it in N is doing nothing. It might let you coast to a stop another 10 ft by putting it in N by having less drag by mechanically disconnecting the tranny from the engine
 

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I beg to differ

All the new high efficency transmisions unlock engine from drive train via tourqe converter once you are in high gear coasting with foot off the excelerator. That technoligy is not new and "does" in fact yield better economy.

I think with the wieght of these trucks "Mass" moving at speed with tires properly inflated they will coast quite a bit further in neutral than they will in Drive. My 2$$
 

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on my 96, the rpms drop to idle the moment you pull your foot off of the accelerator, on my 6.7 the rpms hold until it comes out of lock up.
 

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Haven't owned a diesel truck before and only two automatics before (the wife's minivan and the aforementioned Suburban). But many rentals almost all automatics.

In my experience I've never noticed an auto transmission drop to idle rpm when in D at drive speed just because the foot is off the throttle.

Best practices are always to shift down even in an auto to control speed while descending steep mountain roads. This would have no effect if the engine was not being turned over by wheels through the tranny and TC.

Now with a diesel engine that doesn't get much engine braking (unless you engage the EB) perhaps it might make sense to allow the TC to unlock and the engine to idle. But I can confirm that with the 6.7 it DOES NOT work this way. If you are descending at 60mph with foot off the go-pedal the rpm is at 1800rpm, not 500.

I can also affirm that the engine braking is not strong without the EB on, but it is significant. I guarantee that if you do a rollout test on flat level ground with no wind you'll find that it rolls a lot further in N than in D (until of course it starts to be powered at idle).

So if you are descending a long gradual grade (in my regular travels there is a 7 mile descent that I often do in N) your speed will be higher or your rollout longer in N than D. How much higher? Which speed would be prefered? Probably depends on the steepness of the hill.
 

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Haven't owned a diesel truck before and only two automatics before (the wife's minivan and the aforementioned Suburban). But many rentals almost all automatics.

In my experience I've never noticed an auto transmission drop to idle rpm when in D at drive speed just because the foot is off the throttle.

Best practices are always to shift down even in an auto to control speed while descending steep mountain roads. This would have no effect if the engine was not being turned over by wheels through the tranny and TC.

Now with a diesel engine that doesn't get much engine braking (unless you engage the EB) perhaps it might make sense to allow the TC to unlock and the engine to idle. But I can confirm that with the 6.7 it DOES NOT work this way. If you are descending at 60mph with foot off the go-pedal the rpm is at 1800rpm, not 500.

I can also affirm that the engine braking is not strong without the EB on, but it is significant. I guarantee that if you do a rollout test on flat level ground with no wind you'll find that it rolls a lot further in N than in D (until of course it starts to be powered at idle).

So if you are descending a long gradual grade (in my regular travels there is a 7 mile descent that I often do in N) your speed will be higher or your rollout longer in N than D. How much higher? Which speed would be prefered? Probably depends on the steepness of the hill.
With diesel auto's they tend to down shift on you when you go off throttle thus slowing you down a little more.:thumbsup
 

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The dodge torque converter will stay locked in whenever you enter overdrive. As for driving around town, it unlocks every time you let off the pedal. However i have noticed in my LMM that there is a noticeably greater amount of "drag" than there is in my 600 series cummins. I would call this backpressure myself, but i think we are talking about the same thing. I think this has a great deal to do with the new diesels having a dpf. Regardless of duramax or cummins, we both own a diesel with a dpf and we both are able to tell a difference in drag/backpressure whenever the torque converter is locked in. I am able to tell a big difference if i put the transmission in N when i start down a steep grade. It has to be worth it though, because if the vehicle slows too much then you will in turn waste fuel getting it back up to speed. Also, the truck will tend to shift from 5th to overdrive after switching from N to D, which wastes a little more fuel. As far as whether or not this would damage the transmission, i dont think you are risking it. The only time the transmission is not cooling is whenever your truck is in park. As a matter of fact you should idle your truck in N with the parking brake on if the idle is extended or you parked with the trans hot. I dont think shifting from N to D would damage anything because the torque converter is unlocked when you make the change, and only locks after it shifts from 5th to overdrive. By slowing down on upgrades, and gaining the speed back on downgrades (with the occasional coast in N) i can manage 19-21mpg in my 08 duramax. It is completely unmodified with 300 miles on the odometer. The same techniques in my 600 series cummins can achieve 23-24 mpg. All highway on both figures of course.
 

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I was wrong about my 96' it drops about 500 rpm and then comes down as the truck slows. I kept thinking it dropped to idle but after i posted that i started to second guess myself so i had to go find out for sure.
 
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