If you are doing 2500 rpm at 75 mph (wow, that's tall gearing) then you would be doing a little over 1800 at 55. Quick check shows versions of that engine to have a torque peak between 2500 and 3750 rpm. Possible you were running too low a speed and basically lugging the engine at 55. If you were running close to the torque peak at 75 then that would make more sense to me.
Tom may disagree and that's ok. We all have our own experiences and opinions. Generally I get better economy at lower speeds too. I have had exceptions though. My Datsun was one and my '86 SDL was another. It seemed to like 70-85 better than 60-70. Surprised me too as diesels generally are real rpm sensitive for fuel economy, but other factors can play in there too.
Do you know what axle ratio you have. It should be stamped on a flat surface or a tag on the back surface of the differential housing, usually near the bottom. Hope I remember to look in my data book tonight. This is starting to get interesting.
BTW, Tom is a good guy with a lot of knowledge and I take his information to be well worth considering even if I disagree with it.