When im empty and just commuting: which I have been using my idi for a lot lately...I try to keep my revs as low as possible at highway speeds. 2000rpm tops. Sure, with a t-19 and 3.54's, I have to stay under 55mph but thats fine by me. when people pass me I just look over at them and grin....and people dont know what to think
When im towing I keep revs as high as it takes to keep the heavy smoke and egt's at bay. But im working it then and don't care about mileage. But for just commuting, its slow going for me...I even but some slightly taller tires on it (32's) to keep the revs down another 50 rpm or so.
I have actually never run an idi over 2700rpm, my old 90 with an E40d and 400,000miles on it has, never been over 2300!!! And with my wifes powerstroke, it hauls her horses or whatever around easy and never needs to be wound right up to make a hill.