Was the mileage calculated or from the display? Our truck only has instantaneous, or lifetime average. If I want trip mileage, I have to calculate it. Detuning will definitely help, but if you drive by the boost gauge, you can maximize your mileage, and still keep the higher HP for just in case times. I think you have an OD iShift, so 3.08 shouldn't be hurting you any. Assuming you can keep it in OD, as much as possible. I don't worry too much about day mileage, or even trip mileage, as it typically averages out.