I can get 22-24 mpg on the highway at 65mph or less. I have a 95 f 250 psd, auto, 4.10s. Your city mileage sounds really good to me. I've always thought that less turbo spool meant more mileage. After all a turbo is a restriction and spooling is causing backpressure. But you don't want to lug a turbo diesel you'll make more heat and burn more fuel. My 91 dodge cummins seemed to like no boost at highway speeds better. I swapped to a smaller turbine housing and lost a maybe 1 mpg. At 65 it would hover between 5-7lbs of boost with the small housing verses no boost with stock.
Do you think possibly you were dealing with a headwind on that long trip?
Your right about the people on the diesel truck forums. Its all about what 150hp chip your running. Have you thought about a chip or programmer for mileage. If you bump the timing a little it helps mileage. I'd also run a straight pipe or a donaldson muffler(a BIG one). Maybe an intake too. Thats about it for mileage.
02 Saturn SL
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