Ways to increase diesel efficency?
Usually an EGR delete helps FE on larger diesel engines.
Cold air intake as long as temps stay above 20'F to 40'F, depending on the engine.
Increase the stadic compression ratio.
Water injection.
Belt driven fan delete if you have a belt driven fan and don't tow much.
Installing a larger more modern turbocharger on older generation engines.
A grill block during the colder months will improve aerodynamics and bring up temps quicker.
Unrestricted free flowing, short as possible exhaust.
Then you still have your general vehicle mods like aero improvements and EOC.
Check out the wiki list of mods here:
Main Page - EcoModder