So adding a turbo and doing the fuel line split does nothing for mileage? I figured since the engine isn't working nearly as hard as it was without the turbo, it would increase fuel mileage a couple miles to the gallon, maybe hitting 8 or 9. Am I just not understanding how a turbo works then?
adding the turbo will help your engines responsiveness and give you a better momentum up the hills. the fuel line mod will help give cyl #6,7,8, the fuel that is usually starved in those cyl. the civy versions of the 855 Cummins have had the dual fuel for years..... by giving all cyl the same fuel pressure each squirt of the inj will be the same (providing inj are clean and functioning correctly) then each cyl will perform the same instead of have 3 weak cyl.
As for milage,,,,, the 855 Cummins will likely get 5mpg regardless. the killer in these trucks is that you are usually driving them at redline most of the time due to the extremely low gears in the axles and lack of a multi speed trans allowing you to keep the engine in the oportune RPM range for power versus speed/RPMs.
I just got done adding a turbo and Jakes to mine. If you want questions of these kind of modifications your best bet is the "5 ton Hotrodding" section on here. You wont be slammed as much for trying to think outside the box.