[cleanspeed1]

You made your body essentially into a box and want to keep the utility for work.

Given that you said that you didn't want to drastically affect the looks of the truck, and that you want to get to 16 mpg with an IDI without going into the mechanicals, it may come down to driving technique. The fact that you got 14.45 mpg out a combo that does 10 - 12 mpg most of the time is not bad.

I know you wanted to keep things relegated to aero, but given the constraints and the type of truck you have, it will be hard.

If it were me, and I just wanted to get another 1.5 mpg out of it, and I am not towing heavy or anything like that, here's what I'd do:

Regear the axle. The best cruise rpm is 1600 - 1800 rpms and most of the HDs and Super Duties are geared with 4.10s, 4.88s, sometimes 5.13s. IH intentionally set up the engine to fall off the efficiency cliff past 1800 rpms. Going to 3.55s will make a big improvement with either a stick or automatic. 3.08s would be even better, but without a turbo it will be slow going. ( See Big Dave's Powerstroke pickup and see what he did )

Synthetic lubricant: Front to back, but at least do the rear axle. Lot of drag back there.

Evans Coolant: I am a big advocate of this stuff just because it'll allow you to run hotter, eliminate the need for SCAs ( 7.3 IDI blocks are bad for cavitation when the levels are not proper ), and save the head gaskets. Higher, more stable operating temps = mpg bump.

If the injectors and pump haven't been done, they'll be sorted.