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Discussion Starter #1 (Edited)
Looking for feedback from others who tow similar size/weight trailer. I just returned from a 1000mi trip towing my 5er and the truck (2012 stock LML) got the job done but just didn't impress me holding speed on hills or mileage. I was running 63-67ish MPH the entire trip and on some hills it struggled to hold 60MPH and even downshifted all the way down into 3rd on a couple of grades and just held there pinned at ~55MPH to the top of the grade. I have a CTS and the truck was making boost ~26PSI but just felt weak to me. I was monitoring my throttle input on the CTS much of the trip and just to hold speed on flats I was 45-50% throttle in 5th (I don't use 6th towing...) and on every grade I had to really get into it 80-90%+ just to hold speed.

Having never driven another Dmax with this type load I don't know if my experience is normal or not. Oh and I averaged just under 9MPG hand calculated.
 

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Yeah, get above a 10k camper in the mountains the truck becomes underpowered. It's normal.
 

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Put it in tow/haul and D. You cannot hurt the transmission by leaving it in D (using 6th when the truck sees fit).
Stock tire size?
How much air in tires?
How big of a profile on trailer?

The only reason to go manual is if you feel it shifting 5/6 too often (hunting). Its a diesel. The torque is most usable at lower RPMs than in a gas truck.
 
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Is the 12K actual weight of the camper or what the camper is rated? It is amazing how much stuff we pack and can add weight quickly and weight is a huge factor.
Jim
 

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I also prefer to leave mine in manual fifth when towing heavy. It will normally hold speed up most hills easier. I have an 80hp tow program that works really well and the EGT's stay in check. When descending I will bump it down to fourth if the hill is steep, if the speed limit is 45-55 generally I have to use third. With the turbo brake and engine braking I rarely have to use the brakes during a decent.
 

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Discussion Starter #6
I typically tow in D with T/H an EB on. Trailer is a 30ft 5th wheel Toyhauler with a dry weight of 9k lb and a 12k lb GVWR.
 

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NorCal2500HD.... what RPM are you running?

I find that over 2500 RPM you can go full throttle, it may not shift up a gear, and you burn a lot of fuel and not get anywhere:mad::mad:. Unless you are drag racing, gas engine type RPM is harmful to a diesel. Slow the engine down and let the torque do the work.

I prefer to go in Drive, T/H set, cruise set at 65, and let her run.

If you have to manually control it then easy off the throttle when you can and allow it to shift up. Run it between 2000 and 2500 RPM to get the advantage of the torque a diesel has. In fact if it wants to run 1800-2000 in "D"rive, T/H set, let it do it. If it seems to lug (doesn't respond to throttle up), kick it down a gear so it comes in at about 2300. Makes a lot easier driving experience. Burns less fuel and runs cooler.
 

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More thoughts for you to consider.

Throttle percentage is a product of power needed. Mine with a bit less torque will often run over 50 percent. And on the hills it would go to 100%. Keep in mind there are Interstate roads that will bring you down to 30 MPH. Also slows any other large vehicles, such as semis, buses, and motor homes. If you are down to 30 MPH, don't kick it down into first gear because that unlocks the convertor and heats the trany.

I don't post my fuel mileage because it is never the same twice when pulling a trailer.

There are many variables to the power equation, such as wind, trailer profile, weight and road grade. I think wind speed and direction to your travel is the greatest factor to miles per gallon. Consider you are pulling a unit as high and wide as a semi trailer with only 700 -750 pound feet of torque. Semis may have up to 2200 pound feet of torque and four times the cost. So when they are empty, if you can run with them in the hills you are doing pretty good. Speaking of torque, the Duramax gives peak torque at about 1650 RPM according to GM's specs.

So, you may wish to work the engine closer to the peak torque RPM. I think you may find it will use less fuel. That may not peak the hills at the maximum speed but you may find the saving worth while. If you kick or select it down a gear you move away from the peak torque, and may use more fuel. The Allison trany is super at selecting the correct gear, and that is why we suggest, put in Drive, set Tow/Haul and go. I do drive more by the tach than the speedo until I get up to speed.

I hope this helps you consider how us old time semi drivers have learned to run. Some may call it differently, but we aren't in it to win the race, just get there safe and efficiently, while enjoying the ride.

I don't wish to rain on your parade. We have all been there, done that, and learned from it. Let us know how your next trip goes.
 
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I have an 06 so when towing heavy at a speed of 65 or less I use fifth. Lighter trailers I will let it go to sixth which is about 65 with my truck. My tow rpm is generally 1750-1900 rpm. I have hundreds of thousands of miles towing with this truck and I can generally get 10-12 towing most trailers assuming no head wind. The big triple axle trailers generally are 9-9.5 again assuming no big wind.
 

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What percent grade was it? I've found that over 6%, you will lose speed with 10k lbs or more, but under, you should be able to hold 65 easy. That's with my LBZ on EFI tow tune, which should be very close power wise to your LML.
 

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I have a 35 ft fifth wheel dry weight just over 10k, gvw 13k I have not weighed it yet but with all the crap we have in it (we spend 5 to 6 months at a time in it) so I am pretty sure we are pushing the gvw. This trip we left south fl and traveled to upper state NY pulling some pretty good grades and had no problems with any of the grades. We averaged about 12 mpg for the 1700 plus miles to upper state NY. My truck is an 07 classic so pretty much the same as IGO's . I think you have to find that sweet spot for your truck. I find the same thing as IGO's about 1800 RPM but I use tow haul and cruise to keep it in that RPM range My truck wants to shift into sixth at about 65 so I set the speed to 63/64 MPH to keep it in fifth. If it shifts into sixth it seams to lug down and I see the exhaust temp rise. I think you need to find what works best for you and your truck.
 
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My fiver is very similar, 34 ft, 9.2k lbs dry and 12k lbs loaded. Just finished a 2-1/2 month trip, so we were really, really loaded. I tow in M, w/o using T/H but EB on and had no problems on any grade. I use 6th gear as much as possible and will set my cruise accordingly. But, I don't usually try to maintain speed on steep grades, instead, downshifting as needed while keeping RPMs around 2k. But always had the power to go faster if I wanted to. Going over Monarch Pass I had to make three dead stops for road construction and effortlessly got back up to speed. Hand calculated mileage was 12.5 mpg for the first 4500 miles (which included all the steep grades in CO, UT, OR and CA), but dropped to 12.1 mpg for the 1500 mile run in 100 degree heat from LA to Austin.
 

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I am new to diesel trucks - and to GMC/Chev. This 'tow-haul" position. I understand it locks the torque convertor (presumably to reduce heat generated by the torque convertor's slippage".). I note on the same "stalk" it also has a button to "lock out" the overdrive.

Do I understand correctly, that I can leave it in the tow-hall position ALL the time I am towing, and that it will ONLY lock up the torque convertor - it will NOT lock the transmission out of overdrive ?
 

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Yes, just push the button quickly for tow/haul or hold it in for a couple of seconds to lock out OD.
 

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Tow/haul will:
1) raise the shift points when accelerating.
2) lock the torque converter around 25-30 mph instead of almost 50 mph.
3) more aggressively downshift when braking.
4) be more reluctant to downshift under load (but still will with just a little more throttle).

T/H doesn't lock out 5th (overdrive).

Edit: added #4 observation.
 

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thanks - guys - nice to be in a technical forum where you guys actually know what you are talking about !
 

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Tow/haul will:
1) raise the shift points when accelerating.
2) lock the torque converter around 25-30 mph instead of almost 50 mph.
3) more aggressively downshift when braking.
4) be more reluctant to downshift under load (but still will with just a little more throttle).

T/H doesn't lock out 5th (overdrive).

Edit: added #4 observation.
#4 should be more reluctant to upshift under load
 

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No, I meant the upshifts are delayed (reluctant) in #1. But in addition, it doesn't downshift as early in T/H as it does normally (#4).

For example, say at 55, it takes more throttle in T/H to get a downshift than not-in-T/H. I will sometimes take it out of T/H for a short time to get a more gentle downshift with less throttle when I want to speed up slightly.

I'll also do that for an earlier upshift above 50 into 5th (we're just 5 speed in the LLYs such as 6686L who asked about it)...above 50 because I don't the torque converter to unlock.

If you can go smoothly in/out of T/H without causing any jerking, then it does no harm. (I guess that's my opinion.) I'll probably wear out the switch, though.
 

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The TH programming is design to not make the shifting overly busy and reduces the heating of the trans fluid. You will also notice the TC will lock up and stay locked up earlier. This too reduces the heating of the fluid.
 

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T/H will lock the TC from second up. When T/H is not engaged TC will not lock up till around 50mph.
 
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