I keep the truck in the garage, right now it has been 70-90 day and night, my drive is pretty flat with my cruise set on 70 everytime. About 25miles out of the 30 miles is all highway and my mpg figures are from where I reset the average as soon as I set my cruise on the highway and check it as I am getting off my exit. When I got the 33 and 21 was both the same way, so its not like downhill then uphill back. One was on a Monday then a Tuesday. So I have eliminated all other variables and I shouldn't be seeing that kind of difference in mpg like that. I have talked to a few trusted diesel mechanics and they all came up with is that the truck is trying to go into a regen cycle. I was planning on getting like a 12 or 14ft trailer, since my little 5ft bed isn't going to get much work done, but if I can take that $1500 I was going to spend on a trailer for deletes/tuning so I can get better, consistent mileage, it would be worth it to me, plus never have to deal with filling up the def tank.