I've heard from a few engine builders that the "optimum" RPM for efficiency is the spot where the HP and Torque curves cross. If you have the cam chart of course. Otherwise, a run like that is good once in a while. For me, I like to stay about 2200-2300 for a big block and 2500-2700 for a small block for long runs.

25 Dodge Roadster
66 D100 440/727 (on 72 D100 chassis)
67 W200 360/727
72 IH TravelAll 1110 4x4 345/727/NP205
76 Scout 2 360/727/D20
'08 Dakota SLT Quad Cab 4wd 4.7L