Recent Towing Experience with my Duramax

[RMccord]

Sorry for the delay but here is the promised update on my 3957 miles worth of towing this month. It included crossing allot of desert, the mountains of Colorado and the rolling hills of South Dakota. It had some very long climbs and descents as well as some high winds.
I will preface this by saying this is the largest and farthest I have towed anything and over some very varied terrain.
This will outline how over this distance I determined what I think is the best way to handle towing with my Duramax/Allison combination.
I averaged 12.3 mpg over the entire trip which included about 600 miles of non towing miles.

On my eastward trip from Santa Paula, CA to Hill City, SD I towed the entire time in tow haul mode with overdrive NOT Locked OUT and at a cruise control speed setting of 58-59 mph. With the following exceptions.
When I went over Wolf Creek Pass elevation over 11000 feet I locked out overdrive and in tow haul mode manually climbed the grade in 2nd gear at about 30 mph.
Overall results were good but I noted the following results.
In many places the rolling hills and climbs where sufficient enough to cause the often mentioned downshift from overdrive to 4th. Maybe even some gear hunting by the Allison because of subtle changes in grade enough to make if feel ok to return and then steeping enough to cause it to go back down. This at times was anoying. But still produced a pretty good tow experience for the most part. on the westward return trip I tried some variations as follows. To reduce the hunting for a gear I would lock out overdrive for some stretches. This did ok but I felt it was not as economically efficeint as it could be. I then tried it with tow haul off and overdrive locked out and felt it was a bit better. Finally I tried it as if there was no lockout or tow haul mode. Ironically this I found overall to be the best choice and I think I can explain why.

In tow haul the shift points are raised to assist in accelerating. This is great but it also keeps those shift points up all the time so if you hit a grade (insufficient to cause unit to loose too much speed but enough to hit that higher shift point it downshifts naturally). So I soon adopted the approach of using tow haul only for accelerating from stops and very steep grades to incourage engine to stay in higher gear a bit longer to control rpm's while watching EGT temps.
I also learned that towing at about 61-62 mph was sufficient that on most highway grades including some pretty long ones when towing with overdrive and NOT IN TOW HAUL MOde it would often never downshift and if it did it was almost at the crest of most grades and only for a short duration.
So I finally have adopted the following as my best set up for the Duramax/Allison and towing my approximately 10100# 335rlbs.
1. Starting from stop - Tow haul with Overdrive on
2. When Up to speed - Tow Haul Off Overdrive On
3. Steep passes Tow Haul on and if necessary Overdrive Locked out
Bear in mind my 2005 GMC 2500HD is totally stock with only EGT and Boost gauges added.
We experienced all types of weather and during this trip Tranny never exceeded 185, engine temp never moved off 200. EGT temps never exceeded 1250.
I noticed that the Duramax seems set up so that when the EGT temps reach the 1250 mark it was also when the tranny felt time to shift and the temps would then move right down to about 105-1100. This might be why GM doesn't feel need to have an EGT gauge unit might already know to work within these parameters. Of course chipping unit may take all that away.
I was please with the TV and after setting it up driving literally as if I wasn't towing I found that in most situations it was optimum and produced the least gear hunting, downshifts, lowest rpms and best milage. Further the temps never suffered. The longer I pulled under this configuration the better and less often it seemed to need to downshift. Perhaps this is due to the transmission being smart enough to adapt to the conditions as it is exposed to them. I have been told it learns so maybe so.

I would appreciate any feedback you might have if you try this method yourself.

Thanks
Bob

[padredw]

Bob, thank you for a very helpful report. You asked for comments, so I will address this quote from your post:

1. Starting from stop - Tow haul with Overdrive on
2. When Up to speed - Tow Haul Off Overdrive On
3. Steep passes Tow Haul on and if necessary Overdrive Locked out

I only differ from your practice in your #1. Maybe it has to do with the fact that I have a smaller Montana, but I have never felt any need to have tow/haul on when starting. I get smooth, easy starts without tow/haul, but other combinations of trailer/tow vehicle migh be very different from mine.

Also, I do not have an overdrive lock-out, and have never felt any need for it.

So, in summary, my practice is to ONLY use tow/haul on steep passes (up and down, of course). That means that I only use tow/haul a very small percent of the time. And use overdrive all other times.

One final comment. I never use cruise unless on very level roads. To be fully truthfull, I almost never use cruise control when towing. I'd rather adjust my speed slightly (faster downhill, slower uphill).

We can all learn from one another, and your report is very helpful.

[SAndreasen]

Thanks for all of your information. As you can see from my signature, I have a 315 TT. My TV is a 2002 Chevy 2500HD with an 8.1 l gasser and an Allison tranny. I have found the same shifts as you. I've also noticed that with the tow/haul on the shift point to 5th(overdrive) occurs at 62. I also drive at 58 to 59 like you. Engine runs at about 2200rpm. When driving at almost the shift point caused the least hunting. When driving at just over the shift point, rig wanted to **** all the time when on hills. I got the best gas mileage at the 58-59 mph. TV performed great. Had to look in the mirrors to check if the TT was still hooked up. We were just out to the Black Hills of SD and stayed at the Rushmore KOA at Hill City.

[Montana_738]

RMccord,

Thanks for the report, I have a 2003 Chevy Diesel and have made the approx trip you did last year. I agree with your results.

Bill

[sreigle]

Bob, thanks for a good, honest report. I suspect the reason you saw the change in behavior for the better after towing for awhile is the GM may do as the Ford does. That is, it constantly analyzes all the info the computer receives over the last xxx miles and is smart enough to figure out that it needs to adjust the operating parameters. I see the same thing when towing after a month of non-towing. After about 50 miles of towing the behavior seems to adapt to the new environment, shift patterns change, etc.

[Bowie]

Thanks for a great report--lots of food for thought, even for the Ford drivers. Wasn't Wolf Creek pass a treat??!!! Like to never got our breath back after that one.

[Lady RV]

quote:Originally posted by Bill and Ann

No comment except I may try what you did.

[Montana_1988]

The DMAX/Allison seems to get its best mileage at around 1900 RPM. This equates to about 62 MPH. Break 1900 and your fuel mileage seems to decrease in proportion to the speed increase.

In the rolling hills, the Allison has to work the hardest. It you're going too slow it pulls the hills instead of rolling up them and the transmission does a lot of downshifting. I keep the curise off during these times and try to keep the truck from downshifting. In the Tow/Haul mode I find the truck will pull longer before downshifting if you try to hold it steady and back off slightly as it starts to pull near the crest of the hill. Your speed will drop some but it will pull strong. Remember, we're talking rolling hill, not long grades.

Depending on how I'm loaded, I'm around 21K+ and average between 9.5 to 10.5 MPG fairly consistently. From Colorado Springs, Colorado to Mt Rushmore I ran 10.5 one way and 10.4 the other.

Dale

[NJ Hillbilly]

When driving in the 58 mph range I will shift out of T/H to force the upshift then re-engage it. This way it generally holds the higher gear.

One reason to keep T/H engaged is so the grade braking is utilized more aggressively, this helps when slowing to a stop.

I always tow in T/H even with a 5000lb trailer, the trans tends to heat up faster and higher without it.


John