Mine is on all the time so I can have advantage of the down hill downshift and the brake pedal downshift afforded to me by the haul mode and that added small bit of torque spectrum rpm.
I don't care much about the 1/10th mpg difference I observed over 10,000 mile experiment on my 8.1 Chevy.
The tow haul cruises at the same rpm on flats as not tow haul. That's why the minute to non-existent mpg differences in real life.
Experiment yourself.
As said, some of us drive differently.
You'll get advice from 83mph I'llneverbeadickintraffic folks and 55mph nomatterwhoorwhatorwhy folks.
The debate goes on ad nauseam.
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