Originally Posted by jcanavera
The tow/haul setting on my GM van doesn't change the gearing. What it does do is change the shift points. For all intents it delays the upshifts to keep the engine at optimum revs to give you more power. Likewise the downshifts come sooner and at higher speeds, again to give you more power. For example the upshift to OD when I'm in tow/haul doesn't occur until I get over 50 mph.
This is true, but it does some other things, like raise the line pressure in the trans for more firm shifts and clutch apply pressures. I also boosts the alternator output for better trailer battery charge. In the newer trucks, it also provides for a different calibration for stability control as well as a hill start assist feature. They DO NOT change the gearing.