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Old 10-22-2012, 08:23 PM   #1
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Duramax/Tow Haul/Gas Mileage

I've been towing with the tow haul mode on 100% of the time.

Just curious, when on a flat, straight road at sea level can I or should I turn off the tow haul to reduce RPMs and increase MPG?
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Old 10-22-2012, 08:27 PM   #2
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Tow haul mode is only necessary under any and all conditions, when your GCWR is 75% of max rating or greater. Based on your trailer (unless you're hauling lead in the truck bed) you don't need tow haul turned on at all. Is there more info that would validate a 75% load situation?
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Old 10-22-2012, 08:33 PM   #3
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Tow haul mode is only necessary under any and all conditions, when your GCWR is 75% of max rating or greater. Based on your trailer (unless you're hauling lead in the truck bed) you don't need tow haul turned on at all. Is there more info that would validate a 75% load situation?
Thanks, I doubt if I'll ever be at 75% with my truck (which is why I tow with this beast).

Am I correct that my gas mileage will increase with the tow haul off? I imagine that it must help based on RPM. Actually I'm in love with the Duramax and even the current MPG is not bad.
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Old 10-22-2012, 08:37 PM   #4
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Sure, it will help. You will shift sooner and stay in, respective to speed, higher gears more; and downshift later.

I just got out of my 2500HD Sliverado Duramax demo, unfortunately, and I never used TH mode all summer. I may have punched the button if I would've gone to the rockies last summer, but other than that....not needed. This is explained in your owner manual under "tow haul mode".
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Old 10-22-2012, 08:56 PM   #5
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I replaced my duromax tranny this summer ($5000) after 55000 miles of mostly towing and mostly not in trailer mode. The manual in my '05 truck is less than clear on what happens if I don't use it only the benefits of using it. But now knowledgeable mechanics and articles I have picked up all point to the shifting into and out of OD at cruising speed (which apparently will happen frequently)as the culprit for the destruction of the gears. If you don't want to use trailer mode then hit the button twice to disconnect OD.

I now only tow in trailer mode and see no decrease in MPG. By the way the official reason the dealer gave me was the torque converter was loose...others were quick to add as the result of the OD cycling.
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Old 10-22-2012, 09:01 PM   #6
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I replaced my duromax tranny this summer ($5000) after 55000 miles of mostly towing and mostly not in trailer mode. The manual in my '05 truck is less than clear on what happens if I don't use it only the benefits of using it. But now knowledgeable mechanics and articles I have picked up all point to the shifting into and out of OD at cruising speed (which apparently will happen frequently)as the culprit for the destruction of the gears. If you don't want to use trailer mode then hit the button twice to disconnect OD.

I now only tow in trailer mode and see no decrease in MPG. By the way the official reason the dealer gave me was the torque converter was loose...others were quick to add as the result of the OD cycling.
Hmmmm, don't sound right to me. Even with my 30'er, in the midwest, even in and out of the river bottoms, i only very occasionally dropped out of 6th into 5th. Sumptin' else going on here.
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Old 10-23-2012, 11:25 AM   #7
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I questioned that too (not aware of the shifting). I am not enough of a gear head to argue the point and there may very well be some other assignable cause. But my mechanic had seen this many times before from local farmers who do not use tow mode because they see no reason too and a new transmission is covered by warranty. As for me I will use tow mode until the warranty expires in 3 yrs and buy another.
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Old 10-23-2012, 12:04 PM   #8
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On a flat, level road I think the rpm will be the same in tow haul or regular. On our Dodge the up and down shift points are different, but at speed on a flat road T/H gives the same rpm. Try it and look at your tach. If you stop and start a lot you will probably loose a little mpg in T/H, and it is probably worth it for the better shifting and transmission life. Sometimes in the mountains or in the city I run it in T/H even when not towing. We do not have all those gears that you do in the Allison and I like the way it shifts in T/H in bad mountains.
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Old 10-25-2012, 10:52 AM   #9
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Yes, once in a particular gear, the mileage will be the same. BUT, with TH on, you will stay in lower gears longer and downshift sooner resulting in higher RPMs for longer amounts of time than if TH was off. So mileage will be a little lower....how much....I don't know, never experimented. I'd guess in the flatlands, less than .5 MPG certainly, and probably less than that.
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Old 10-25-2012, 08:05 PM   #10
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I tow at about 62 indicated (true 60 1/2 according to my GPS). In tow-haul, it will not shift into 6th gear at that speed. Accordingly, I always stay out of tow-haul except in very hilly country. On flatter country, I can stay in 6th for long periods, so there is a minimum of shifting in and out of 6th. BTW, even 5th is an overdrive.

In hilly country, running, always, in OD, I shift into "M" and use the button to downshift when I sense that I am nearing a shift down. That shifts early enough that the shift can hardly be felt and I never get into a position of getting a double shift down. As I near the top of a hill, I use the button to shift back to 6 and let the truck decide when to make the shift. I get the best fuel mileage that way and it gives me something to do.

I have had two Durmax/Allisons and never a transmission trouble.
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Old 10-25-2012, 08:31 PM   #11
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Though I Would Normally Agree with dznf0g...

...on just about everything, I must disagree on this one.
What I loved about my 5-speed Allison, and what I love even more about my 6-speed Allison, is towing in the mountains in the Tow-Haul Mode.
Otherwise, in the wide open spaces, I don't use Tow-Haul because it tends to hold me in lower gears too long. But when I am in the mountains and descending(using Tow-Haul), I love the way the transmission down-shifts and provides engine braking. All I have to do is steer.
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Old 10-25-2012, 09:30 PM   #12
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...on just about everything, I must disagree on this one.
What I loved about my 5-speed Allison, and what I love even more about my 6-speed Allison, is towing in the mountains in the Tow-Haul Mode.
Otherwise, in the wide open spaces, I don't use Tow-Haul because it tends to hold me in lower gears too long. But when I am in the mountains and descending(using Tow-Haul), I love the way the transmission down-shifts and provides engine braking. All I have to do is steer.
Um, I don't think we disagree...way upthread (post #4) I discussed using it in the mountains. All my other comments were relative to the flatlands around the midwest.

Also, we have 30'ers....op has a 27'er....maybe 1500 lbs difference? Still in the mountains, he may prefer TH mode. BUT, owner manual doesn't even make that distinction, IIRC.
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Old 10-25-2012, 09:48 PM   #13
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I get the best fuel mileage that way and it gives me something to do.
I'm with you, I often manually shift just to keep my hands busy, kind of miss having a manual on this truck other than that not too much to complain about.
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Old 10-25-2012, 09:50 PM   #14
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Uggg, I don't want to talk manuals right now. I have a 99 Tracker transmission on the floor of the garage right now. I don't mind fixing stuff, but I'm doing this just to sell it. I'm getting too old for this rolling around on the garage floor manhandling transmissions.
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