NEWBIE!! i need some help...04 Avalanche

I have a 2004 avalanche.180, XXX miles. I have a few trailers i pull, one being a pontoon boat. My truck has lost power in her old age. so i bout a unlock diablo sport predator. I am new to this and no instructions dont help.. I started playing with it and off the bat entered the wrong stuff... I put my stock tire size was 295 70 17 which is my actual size... can i change this? what other settings do i need to change... i dont mind running 93 oct if it will help... looking for pulling power and speed if possible... this is my first tuner so I dont really understand this stuff.