Air entrainment effects are avoided in a bi-directional, reel-to-reel tape transport in which magnetic tape can be moved in either of two opposing directions for data recording thereon. Control of tape position is implemented in a servo algorithm that uses tachometer inputs to determine parameter values for generating reel motor drive currents. A fine-line tachometer is mounted on each of two reels in the tape drive and multiplexing selects the fine-line output from the tachometer on the reel supplying tape for use in the servo algorithm. At least one of the tachometers is preferably of the type which generates an index line once each revolution of the motor to which it is mounted. This tachometer is coupled to the reel to which the tape is initially threaded and the index line is used to indicate that the motor is at a predetermined threading position for this operation.