To be honest, it is almost something that you learn based on the terrain you are wheeling in. Each area is different and requires a different scenerio. As far as going up steep grades I would downshift to a lower gear and same goes for going down a steep grade. Let the engine do the work, not your brakes. We have all sorts of road signs here in Colorado that remind use "Hot Brakes Fail" and it is true. The O/D in my opinion is more for towing when you want less strain on your engine with all the weight you are hauling. But I do use mine here and there going up a moutain pass on the highway just to make it easier on the engine. Oh yea, if you do decide to downshift going down a hill, make sure to slow your speed before just shifting otherwise you are going to dump your transmission. So if you are going a good speed in "D", slow down a bit before going into "2".