What oil are you currently running in your truck? Changing to a thinner oil can help to reduce efficiency losses do to friction. Another member has reported great gains in his 3/4 ton 6.7L cummins diesel by switching to some
Amsoil Series 3000 Synthetic 5W30 Heavy Duty Diesel oil.
Diesel Dave: "The biggest change recently which put me over the top was switching to some 5W-30 synthetic oil, which has bought me another ~5-10%."