I have been on all the 6.0L forums for a lot of years. This was a VERY active topic in the early years.
First off, there is very little you can do in the form of mods to improve fuel economy....and a PCM tune and cold air intake will not help. The Ford OEM (stock) intake is about the best you can get. Most aftermarket intakes are not as good, and the OEM design pulls air from in front of the vehicle, so it is a cold air intake already.
The only mod that I have seen have a favorable impact for some is the economy FICM tune (from FICMrepair.com).
After that it is maintenance and driving style!
1. Make sure the brakes aren't dragging. Test with an infrared temperature gun.
2. Keep the tire pressure up.
3. Make sure the A/C clutch isn't sticking
4. Some people claim that running 10W30 diesel oil can help a little.
5. Keep your EGR valve clean (also clean the intake around the EGR valve).
6. Keep the pressure sensors accurate (make sure EBP, MAP, and Baro read the same at KOEO).
7. Keep the temperature sensors accurate (IAT1, IAT2, EOT, and ECT should all read ambient temperature after 12 hours of letting the truck sit without being started)
8. Make sure you don't have any boost leaks (turbo up pipe and y-pipe)
9. Make sure you don't have any intake or Charge-Air-Cooler (some call it the inter cooler boots) leaks (boots OR cooler).
9. Old injectors can impact fuel economy by several mpg. Trouble is, it costs $2000 to replace old injectors. Tough to break even very quickly.
Some people have said that fuel additives help, but I have never noticed it. The cetane level here in the U.S. is pretty low unfortunately, so adding a cetane booster can help with the efficiency of combustion (ie less soot).
The biggest help is driving style.
1. Keep speed around 60 mph for best fuel economy
2. Anticipate stops (coast into them if possible) and do not brake excessively
3. Minimize excessive accelerations. You use quite a bit more fuel to get turbo boost.
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