I sold my 21 7.3l and ordered a 6.7l. I was averaging 11.2 MPG and worse when towing six months out of the year. It’s a great motor and will pull anything, however the transmission was very erratic, I could barely hook a trailer up by myself. It lurched forward and backward when shifting. I was told that it was because of the torque of the 7.3. Not sure but does seem like a common complaint on the Tremor forms. I test drove a 6.7 much it’s smoother. Overall both great choices.