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Old 04-20-2015, 01:56 PM   #15
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From page 9-91&92

Tow/Haul is designed to be most
effective when the vehicle and
trailer combined weight is at least
75% of the vehicle's Gross
Combined Weight Rating (GCWR).
See “Weight of the Trailer” under
Trailer Towing on page 9-72. Tow/
Haul is most useful under the
following driving conditions:

When pulling a heavy trailer or a
large or heavy load through
rolling terrain.

When pulling a heavy trailer or a
large or heavy load in
stop-and-go traffic.

When pulling a heavy trailer or a
large or heavy load in busy
parking lots where improved low
speed control of the vehicle is
desired.

Operating the vehicle in Tow/Haul
when lightly loaded or with no trailer
at all will not cause damage.
However, there is no benefit to the
selection of Tow/Haul when the
vehicle is unloaded. Such a
selection when unloaded may result
in unpleasant engine and
transmission driving characteristics
and reduced fuel economy. Tow/
Haul is recommended only when
pulling a heavy trailer or a large or
heavy load.

This may describe the behavior you are seeing. Good luck.
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Old 04-22-2015, 10:13 AM   #16
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Gary,
Thanks for your post. You're right about that quote…it was in a different place in my manual.
Odd that the page does not show up in either the table of contents or the index…and there are similar errors elsewhere in the manual.

"This may describe the behavior you are seeing. Good luck."
I'm not sure if you are replying to my (former)issue or Jack's(the OP)?

Are you saying that neither of us needs to engage T/H mode with our AS's?
In either case, I believe we're both well above the 75% threshold…

I'm also puzzled about the transmission temperature issue. I believe that an important concern when considering a tow vehicle is trans. cooling capability.
As I mentioned before, I notice a considerable increase in trans. temp. when towing without T/H mode, which seems to just ask for trouble.

Any thoughts?
Thanks again,
Dave
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Old 04-22-2015, 10:58 AM   #17
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I thought since I've now had the truck looked at by a dealer I would provide an update. We are on the road in Colorado and I visited King GMC in Loveland. I've had them do routine service on our earlier Suburban during previous visits here. They are first rate by the way. My truck is a CPO so still covered under warranty, so no cost to me whatsoever.

Anyway it turned out to be a reprogramming of the transmission control module. On my way back to St. V'Rain State Park this morning, I manually downshifted from 6th to 5th and all seems good. Thanks for all the feedback here.
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Old 04-22-2015, 11:50 AM   #18
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Great outcome. Safe travels.
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Old 04-22-2015, 02:35 PM   #19
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Enjoy CO! We've had a nice rain and it's green!
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Old 04-22-2015, 03:06 PM   #20
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I have owned an 2003 and currently drive a 2012.

Both downshift from OD a bit firmer than other cars/trucks. Especially with tow/haul activated.

Regards,

JD
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Old 05-04-2015, 12:41 PM   #21
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I have a '15 Duramax towing a '15 Classic. Through the mountains of Vermont & New Hampshire, I enable T/H only while descending hills which works amazing with the exhaust brake to keep the set speed.
If I keep T/H always engaged; she'll have a tendency to stay in 5th gear longer than I'd like.

In the city, I always keep her in T/H.
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Old 05-04-2015, 02:20 PM   #22
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I do the same thing with Tow/Haul as Ron. On the highway exhaust brake is on and not in T/H. Actually seems to improve MPG since you are in 6th instead of 5th gear. In traffic, either on the highway or in town typically use T/H as you get better down shifting if you need to slow down.
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Old 05-27-2015, 03:12 PM   #23
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I am considering the 2015 2500 Duramax, can anyone comment on MPG?
Thanks
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Old 05-27-2015, 03:29 PM   #24
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towing a 2015 Classic, 12.5-13.5 range. Around town 16. Highway no towing 22-24.

And regular and diesel are the same price at the pump here in Erie, CO. $2.49
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Old 05-27-2015, 03:44 PM   #25
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Quote:
Originally Posted by ghaynes755 View Post
towing a 2015 Classic, 12.5-13.5 range. Around town 16. Highway no towing 22-24.

And regular and diesel are the same price at the pump here in Erie, CO. $2.49

40% deduction isn't bad. It's within the usual 30-40% range. Same speed towing and solo? Use cruise in both instances? 14-15 is possible if so, would be my guess.
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Old 05-27-2015, 04:33 PM   #26
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Tow at 65 and use cruise most of the time. Actually much better mileage than my previous 2014 5.3l Sierra.
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Old 05-27-2015, 06:17 PM   #27
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Our 2012 hits around 12MPG towing... on nice and flat running around 55 indicated, but close to 57-58 actual. When climbing hills (small Texas type) I hit around 9MPG.

Towing a 34' and usually loaded with headwinds..
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Old 05-28-2015, 01:49 AM   #28
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Another data point... but not towing, we've been averaging around 17.5 mpg with mixed around town and highway driving during the first 1300 miles driving a 2015 Sierra Denali HD Duramax/Allison. And we LOVE this truck! The engine is surprisingly quiet, surprisingly powerful (fast!) and the features/creature comforts are great.

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