GM used to recommend using the tow/haul on the Allison backed 6.6 and 8.1 engines anytime you're towing 7500# or better. I use mine anytime I'm towing, I don't use it running empty.

I can't speak for the GM or Dodge version, But the newer Ford version ( 2003 and newer Torqueshift) was designed to be used anytime you wanted the shifts points to move to RPM bands that worked better for an engine under load.  Under light acceleration it holds the gear a little longer, this keeps the engine in the powerband. On my F350 this means it may stay in a gear up to about 2200 rpm with the Tow/haul off it would upshift at 1800 rpm. The tranny still uses ALL the gears, It does not lock out the overdrive like it use to on the Pre-2003 tranny's.

An example of this is when I come through town. The speed limit is 30 mph. With the Tow/Haul turned off. My truck loafs along at about 1100 rpm in 5th gear at 30 mph. If the traffic starts to speed up the tranny downshift to 3rd gear as I press on the accelerator. As soon as I take my foot off the accelerator, it again upshift back to 5th gear and 1100rpm.  With the Tow/Haul engaged, it holds 3rd gear at 2200 RPM all the way down the street ( regardless of whether I'm touching the accelerator). The engine stays in the powerband and the tranny is not shifting back and forth so much. It also forces the Torque convertor to stay locked up when you decelerate ( for engine braking) at speeds above 25 mph.

The old EO4 Trannys prior to 2003 just locked out overdrive when you pushed the Overdrive button. This would force the truck to stay in gears 1-3 at highway speeds. but did nothing to the gear pattern at lower speeds where OD would not usualy engage.

So to answer your question, I use the Tow/Haul anytime I want to keep the truck at a litte higher RPM or keep the Torque Convertor locked up. Regardless of whether I have a trailer hooked up or not. An example of this may be in the mornings when I pull out of my neighborhood. Which has 25 mph speeds.  By pressing the tow haul, I can foce the tranny to hold the lower gear forcing the engine to sligher higher RPMS, forcing the truck to warm up a llttle faster.

The answer depends on your use.  IF you have a heavy trailer...  More than say 7500 pounds as Retento quoted...

And you're pulling hills or Stop + Go, then yes keep it in Tow/haul mode.  The Allison does more than move the shift points.  It locks the torque converter to prevent slippage.  The lock-up makes for a harsh shift but it keeps the transmission cooler and reduces the engine rpm in gear.  (see for your self... shift into t/h when you're pulling a hill in 4th gear.  the engine rpm will drop as the torque converter locks.  Shifting out of t/h will do nothing as the TC is already locked)

If you're just cruising on the flats even with a heavy trailer, normal mode will shift smoother and sooner... saving a bit of fuel

This is how the Dodge is set up also, but I leave my on TOW/HAUL when loaded and the main reason being the Dodge C/C tow/haul is also the od off buttom and when you go from tow/haul and hit the button again it goes to od off then hit it again to regular od and I don't like the tranny down shifting that hard while loaded.  Plus when we have a trailer hooked up it's either the 3 horse Sundowner 8010 (9500lbs) or the 28' flatbed gooesneck (4000lbs) empty wieght.

I think GM has just tow/haul. (not sure) have only used sister inlaws 1 time.

My GM 8.1/Allison won't shift into 5th gear, in tow haul, until I get to about 65 mph. Knock it out of tow haul and it wil shift into 5th at 45 mph. Someone came up with/invented a switch that would lock 5th gear out until you were ready for it..... An aftermarket type deal. The newer trucks may have this already, or it's built into the gear selector... PRN654321... Place the shift lever into the highest desired number and that's as high as it will shift.  Mine is an old 2001 PRND321 model!! D gives you 4th and 5th.

Hey Reteno... aka Leadfoot.  Let up on the gas a bit and the Allison will up shift into 5th before 65. 

I've taken to pushing the T/H button when the Allison downshifts for a hill.  It locks the TC and makes for a cooler tranny.  (By my tranny temp gauge) I first tried this on the Fancy Gap climb.  Then, after the climb turning the T/H off to get a quick up shift. 

LOL!! I know what you're saying, about letting off and let it shift, I'll take it out of t/H going down the other side just so it will go into 5th sooner...... Friend of mine thought my truck had a manual trans.... I'll watch the tach and listen to the engine while it's shifting up through the gears, I'll back off just before it shifts, let it shift and get back on the throttle. Sounds like I'm shifting a "Roadranger". Have you learned to split the shifts with the T.C., using the tow haul button, between 2nd 3rd 4th and 5th??!!! I can get nine gears out of that 5 speed Allison!! LOL! Works good if you've dragging that 6H2H, "Super Chief" behind you.

Good grief, all the thinking in this thread is making my head hurt. 

If I wanted to worry about what gear I was in, I would have bought a truck with the ZF-6 and not the Allison.  True it says somthing about only needing the T/H mode with 75%+ of the GCWR, but when I hook to a trailer of any size I push the button with the picture of trailer on it.....now I can worry about more important things, like beer.

If you are going to worry about beer with the tow/haul on, does that mean you are drinking and driving?

Maybe you should get you some beer and give the forum a break for a while.

But do not drink and drive!

Well only if you bought your Dmax before 2006...because the ZF6 has not been available since then...

01-03 was the LB7 Dmax with 300hp/520lb ft of torque it had either a 5speed allison or ZF6 stick shift

04-05.5 was the LLY Dmax with 310hp/605lb ft of torque it had either a 5speed allison or ZF6 stick shift

05.5-06 was still the LLY Dmax but they had the 6speed allison or ZF6 stick shift these are sort of rare.

06-07 was the LBZ Dmax with 360hp/650lb ft of torque had the 6speed allison no ZF6 stick shift

07.5-09 is the LMM dmax with 365hp/660lb ft of torque has only the 6speed allison.

Having t/h on or off while driving the flat freeway will make no difference in mpg. 

When not in t/h, once above 45 mph and in 5th range the t/c will lock.  As opposed to using t/h when the t/c locks at ~25 mph in 2nd range.  Either way once to 'crusing' speed youll be in top gear with the t/c locked = best mpg.

Well, I like my tow haul mode for when I drive in town with lots of stoplights. It shifts down and slows the truck and trailer with less brake usage.  I love it for that, as well as up and down hills.  If I'm on the flats I turn it off.

Amanda