Let's say you just took delivery of a used 27000# trawler powered by a
single Cummins turbo-diesel 220, model 6BT5.9M. You want to get maximum
miles per gallon out of it and know the slower you go, the better, but
these motors don't like to go slow for a number of reasons. Of course,
you also want it to last forever because it might cost $10,000 to
rebuild it, not including pulling it out and putting it back in..

What would you consider a minimum cruising speed, with an occasional
shot at 2000 rpm just to clean things out?

Capt. Jeff