A hydrostatic transmission on a tractor is operated by a microcontroller providing current commands to the hydrostatic transmission pump, and manual controls that provide signals to the microcontroller so the current commands cause the hydrostatic transmission to operate in either a first mode or a second mode. In the first or automatic mode the swash plate position may be continuously variable based on the position of a foot pedal. In the second or gear mode the swash plate may be moved to any of a plurality of discrete positions based on the position of a manually operable switch.