towing mileage of trucks

I believe all of the diesel engines have similar fuel efficiency. The only thing that may make the newer engines appear to get worse fuel mileage as compared to earlier models is the increased hp and torque ratings. If one uses that energy, the fuel consumption will increase to develop the power. If one very conservative and does not have a lead foot the old vs newer engines will be similar. It get rather complicated and I suspect the newer Ford 6.7 vs the older Ford 6.7 is not as economical over-all, but they do produce more power which translates to higher fuel consumption. Marketing is better for more power these days than economy. Now days it is more of a macho thing than necessity.