Tow Haul.......When?

APT wrote:

wintersun wrote:
GM's engineers recommend using Tow Haul mode when the total load is greater than 75% of the GCWR for the truck.

Except that a 4000 pounds high wall RV is harder on the truck than a 10000 pound flatbed or boat.

Based on what calculations? Force on a trailer is equal to the area times the wind pressure per square foot times the area times the drag coefficient.

With a 60 sq. foot trailer at 60 MPH and using a Cd of 1.0 (like a brick) the force involved is 540 lbs. which is hardly the 6,000 lbs. increase you have stated. Even at 80 MPH the air drag force the tow vehicle needs to overcome with a completely square trailer is 983 lbs., which is not a lot for most tow vehicles to handle.

If the tow vehicle is a mini van pulling a 5,000 lb. trailer then the air drag is going to be a factor. With my truck that has a towing capacity of 17,300 lbs. this does not come into play at all.

This does assume that you are interested in the facts and not simply making things up as you go which has become so popular with politicians and Fox pundits and their cohorts in crime.



That depends on whether you are trying for a speed record and towing at 80 MPH and the grade. Going up a grade at 55 MPH I would much rather be towing a 4,000 lb. hard side trailer than a 10,000 lb. flatbed.

As soon as I hitch up the RV I use tow/haul.

wintersun wrote:
GM's engineers recommend using Tow Haul mode when the total load is greater than 75% of the GCWR for the truck.

Except that a 4000 pounds high wall RV is harder on the truck than a 10000 pound flatbed or boat.

Tow Haul mode is designed to minimize heat buildup in the transmission be changing shift points and also keeping the torque converter locked longer before upshifting. GM's engineers recommend using Tow Haul mode when the total load is greater than 75% of the GCWR for the truck.

If your truck weighs 6,000 lbs. and has a GCWR of 20,000 lbs. then Tow Haul would be engaged when the trailer load exceeds 10,500. My 2011 GM diesel truck weighs 7,200 lbs. and has a GCWR of 29,000 lbs. so Tow Haul should be engaged when I am towing more than 14,550 lbs. per GM's recommendation.

Some people mistakenly think Tow Haul helps with overall performance when towing and it does not. Many also take the transmission out of Auto and manually do downshifts and with GM trucks this shuts off Grade Control entirely.

"What about the fact that you can see where the grade ends or if there is another one coming and the "smart" truck cannot?

I often turn off T/H briefly because it is very aggressive about staying lower gears (usually a good thing), even when the hill has let up for good."

A good driver can anticipate what the upcoming conditions will be. In order to save fuel, I will turn off the T/H and take advantage of the free energy that would have been wasted by not letting the truck coast. Obviously the speed may increase a bit and safety comes first.

With my 2005 Duramax/Allison, if I am hitched up to our 12K 5th wheel, I put it in T/H mode and forget about it; flats, uphills, downhills, and have had no complaints/issues. I even sometimes use it when not towing if on steep mountain roads, it keeps the tranny from hunting needlessly between hills and flat areas and saves braking; has been great for that too.

Hoppypoppy wrote:
Help me understand.....if I am in T/H, I can forget about the "O/D Off" feature? The T/H will automatically make whatever shifts are necessary? Also did I understand correctly that the Allison will not kick in if O/D is off??? Trucks 12 years old and I am still learning stuff. Love it!!

What truck do you have again?

I guess I can't speak specifically for the earlier Duramax/Allison combos, but with my '06, I would turn on T/H, put it in "D" and that was about it. The transmission did an excellent job controlling the shifting. No hunting, no hanging up, no issues.

Hoppypoppy wrote:
Help me understand.....if I am in T/H, I can forget about the "O/D Off" feature? The T/H will automatically make whatever shifts are necessary? Also did I understand correctly that the Allison will not kick in if O/D is off??? Trucks 12 years old and I am still learning stuff. Love it!!

You can still use O/D off in T/H mode. One consequence of T/H mode is not only preferring a lower gear longer, it will also wait to downshift to minimize hunting, i.e. you can "lug" longer in higher gears. If you find yourself in O/D on a long light grade and don't like how far you have to push the pedal, instead of pressing down more to cause a downshift you can just click O/D off and leave it that way if the terrain means you likely won't need it for awhile.

The computer can only do so much with what you're currently doing. You have the advantage of knowing the upcoming terrain. Leaving O/D off can further cut down the number of shifts in rolling terrain.

I rarely use T/H. The majority of the time it keeps my truck in a gear (or two) lower than needed. I'll use it on a big downhill grade,but switch it off afterward.

Help me understand.....if I am in T/H, I can forget about the "O/D Off" feature? The T/H will automatically make whatever shifts are necessary? Also did I understand correctly that the Allison will not kick in if O/D is off??? Trucks 12 years old and I am still learning stuff. Love it!!