I have a 2004 Silverado with a 4 speed tranny and a 5.3 engine. I love the truck but I hate towing my boat with it. If I keep the cruise OFF, it will do fine and I get about 13 mpg. If I leave the cruise ON, it will down shift to 2nd on almost every hill and shoot my rpms up to almost 4000. My mpg suffers and I get about 11 to 12 mpg on the same trip. I have tried Tow Haul, running in overdrive and without overdrive. It doesn't matter. It seems if I can prevent the excessive down shifting by driving with the cruise OFF, then there ought to be some way to de-activate the downshift to second while in cruise. Any ideas?