My overhead is approximately 1 mpg to the optimistic side, and I found no significant change to accuracy by adding the mp8. Overall, I think in an otherwise stock 6.7 the mp8 will gain about 2mpg. I think the mp8 has the ability to gain at least 3-4 mpg, but due to more frequent regens it won't gain all that with a dpf. With a dpf delete you will gain the full fuel economy gains from any "chip" due to too not having more frequent regens from more soot. Trust me, after adding a 5" turbo-back exhaust, the mp8 will lay down a nice cloud of smoke with it turned up compared to a little puff with it off. Assuming I won't have problems ahead, I feel the mp8 has paid for itself already, I basically get an extra 100 miles on a tank with it.