How do different RPM–MAP settings affect range?
The advantages of oversquare (MAP > RPM/10) are:


  
  less vibration, noise, heat, and wear
  longer engine life as a result of this
  much less noise emitted from the engine/propeller
  more efficient cylinder charging and better combustion, gives lower fuel consumption if leaned properly
  less power lost to friction inside the engine
  
  
  — Source


Since there are fewer loses to friction and high propeller RPM, the same fuel flow will yield slightly faster flight, and thereby range.



Another article from AVweb that discusses the topic: Why "Over Square" Is Good