Yeah, like Marty said, you need to put some guages in there. I ran a 5" straight pipe on my 7.3 with no problems at all. You will be fine with the 4" straight pipe. Alot of your power numbers will depend on what chip/programmer/module you go with. Good Highway running if you keep your rpms below or arond 2000, and dont get on it a whole buch, I would say anywhere between 16-18mpg. And yeah, with a power adder, you will beable to blow all the coal you want.