[Festus2]

If you have the Allison transmission, I think that most people simply leave it in tow haul whenever they are towing. This is what I do and it works for me. The GMC truck manual also recommends that you leave it in this mode. I don't have the electronic overhead console which gives you the various readouts so I have to figure out my mileage the "old way". I also use the overdrive cut-out feature when I am going down a long steep hill as it seems to hold back better. Because your truck is a newer GMC, it probably has the 6 speed manual option in addition to the auto 6 spd. Mine is a diesel which adds another variable to the equation.