Pump selection (for new or replacement) requires the engineer to calculate the "system curve" - the relationship between flow and head (both friction and static) imposed by the piping network and the source and destination tanks. The engineer must then review manufacturer "pump curves" - which describe the relationship between flow and head provided by the pump. Pump curves also show an overlay of efficiency curves.The engineer's goal is to find a pump that provides the desired flowrate at the associated system curve head. This "operating point" is where the pump curve intersects the system curve. That operating point can occur at a 45% efficiency area or at an 85% efficiency area. By changing the impeller size or the pump size the engineer can find an operating point that is "the most efficient."Folks used to do these calculations by hand, but now we have computer programs. I used Cameron's Hydraulic Data (17th edition - 1988) to do all my work. The book is still published today (2002 copyright). See Amazon link here.