i just recently bought a 97 flatbed dodge with only 125,000 miles. i have set my valves and made adjustments on the pump. i boosted the waste gate up and im still only getting 15 miles to the gallon. The truck does have 410 gears but i only drive it at 60 miles an hour.
I am like Buck2 tell us all you know about the truck that will help. And adding some timing will help. We need to know things like ride height, 2 or 4 wd, tire size, 5 speed or auto.
Now I really doubt the truck really has only 125,000 miles on a 14 going on 15year old truck. That would be less than 10,000 miles per year. It is possible but doubtful. Usually I would start out saying
Tab the killer dowel pin or make sure it has been done.
Check your brake pads.
Check your wheel bearing.
Next Get a better air filter. (BHAF)
Next Get gauges. Pyro, Boost at the least.
Next Set your timing to 14.5 degrees, maybe a little more.
Next Get a 4 inch turbo back exhaust system.
Next Get a 4k GSK (Don't rev above 3200 without new valve springs 60 lb or better).
Next Get a set of good injectors, by now yours are old enough they need the pop pressure set and may need rebuilding you can get a set of 5X12 for what it will cost to repair yours.
There is more but that will keep you busy.