Abstract: An exemplary pump jack system (10) is operable to cause well liquid to be pumped to the surface by a subsurface well pump (29). The exemplary system includes a driver (42) that is in rotatable connection with a pump/motor (48). The pump/motor is a rotary type variable output positive displacement pump/motor that delivers working liquid at a selectively variable flow rate in closed liquid connection to a motor/pump (38). The exemplary motor/pump includes a rotatable output coupler (40) that is in operative connection with a gearbox (34) which causes the pump jack to undergo reciprocating motion. The motor/pump is a rotary type positive displacement motor/pump that is in closed liquid connection with the pump/motor liquid inlet of the pump motor. The pump/motor is operable to deliver working liquid flow therefrom to drive the motor/pump at a selectively variable speed while the driver operates at a generally constant rotational speed.