over drive - a lot of guys say to not even pull anything in overdrive, but i think on all the newer trucks it is ok, but i am not 100% sure. I have an 03. And i pull in OD, but it all depends on the road, hills, speed limit, and wieght. I wouldnt worry to much about OD. just dont lug the heck out the truck. And back to milage, i would say thats ok on mpg, but i thought the 6.0s were suppost to do a little better than that..... A guy that hauls for us gets any where between 10-14 pulling 14-18k lbs. But that was before his truck started all the famous 6.0 problems.