We use 8" creosote post around here. Most of the time we do like you are talking and use a tractor with a post hole digger near 4ft deep. We put some concrete in the bottom of the whole before putting the post in. This performs 2 duties as a footer & to prevent water penatration from the cut end of the post. We pack in quikcrete around the post. Have some that have been up 20+ years and are in good shape.
