An HST propel system includes first and second pumps having displacements variable in response to movement of first (31) and second (33) control shafts, respectively; and a linkage arrangement for moving the control shafts in unison in response to movement of an operator input (37). The linkage arrangement comprises a crank arm (35) rotatable about its axis in response to movement of the input device, first (41) and second (51) input arms fixed relative to the crank arm, and first (45) and second (55) elongated control rods pivotally connected to the input arms. The system includes a swivel connector (43) cooperating with its input arm to define a swivel axis (S.A..sub.1), fixed relative to the axis of the crank arm, and a swivel connector (53) cooperating with its input arm to define a swivel axis (S.A..sub.2), moveable relative to the axis of the crank arm for straight tracking adjustment.