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Obviously, in the automatic, there is a hyraulic pump that needs to be powered to operate the clutches, and cool and lube the clutches and bearings. When you are driving in a certain gear, one or two multiple plate wet disc clutches are engaged, and several others are disengaged. In the clutches that are disengaged, the plates are a few thousandths of an inch apart, sliding past each other, with oil flowing between them for lube, all of which makes a certain amount of heat, also, and heat = wasted power. If you question that, keep in mind 'most all automatics need oil coolers of one type or other, to reject this extra heat from the oil, while, generally, manual boxes need no oil coolers. If you compare identical vehicles, one automatic, and one manual, I think there still is a measurable difference in fuel economy. While it is not as great as it was in the days before the 4 (or more) speed automatics, with locking convertors, never the less, it does exist.
As for the injection pump, the early 6.2's generally got pretty good mileage at light load. As time went by, and they screwed up the pump for more HP, and turbo'ed them, the slots in the cylinder head injector pre-cups were enlarged to accomodate the larger volume of fuel burning at each combustion stroke. It is the opinion of many that this change lowered light-load fuel economy.
As for going back to the mechanical pump, first off, you'd need a stand-alone controller to operate your electronically-shifted transmission. These cost $800 to $1500, unless you can find one someone is trying to unload. I have had quite a few older units with the mech. pump. A neighbor had a 1993 2500 with a mech pump, and I have a 1994, with the electronic pump. He bought his new, mine had about 54,000 miles on it. My '94 got AT least as many MPG's, and started better cold, and had more power, and was more responsive. I can see NO reason to go back to the old mech pump. The electronic units initially gave some trouble. Most of those pumps have been swapped out for improved units. In addition, mounting the pump's electronic box, the FSD, on a remote heat sink will cure 'most all the remaining problems. The electronic pump is able to maintain much closer control of desired injection timing, based on RPM's and load than the mech pump, which should be good for mileage. There are a few "chips" in the market for these pumps. I don't know much about them. Perhaps SOMEONE is making a chip that will give slightly better light-load economy?
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