rickjo, interesting math. 6.0s didn't use DEF. How much would you have saved if you had kept the 6.0? Btw, my 6.0 never got worse than 12 with TC, and 18 without. You didn't say how much you ride empty.
I traded my 6.0 for the 6.4 F450 because I wanted better brakes, 19.5s, and overall a better camper hauler. I wouldn't say it was a practical move, but I like it unless it is empty. ;). But, I don't drive it empty much.
I think the math is probably a wash compared to the desire to have one or the other or as Northside puts it, smiles per gallon.
