Greetings John,

I picked up MPG by doing two simple things. Set the valve clearance back to spec, and checking and resetting the start of fuel delivery on the no. 1 cylinder. Was getting 23mpg before, now average 26-27.
Just a thought. If there's black smoke coming from it's behind, I'd check start of fuel delivery. It's also a good idea if you haven't run a can of diesel purge through the injectors, to do so. It will also help improve your spray pattern, hence burn rate increases and so does fuel mileage.

Charles