A drive system for propelling a power machine over a support surface is disclosed. The drive system includes a hydraulic pressure source and a drive assembly. The drive assembly has a hydraulic motor in fluid communication with the hydraulic pressure source that is switchable between first and second displacement arrangements. A shift valve receives pressurized fluid from the hydraulic pressure source and provides a shift signal to the hydraulic motor to selectively cause the displacement of the hydraulic motor to shift between the first and second displacement arrangements. A sensing device capable of generating a load signal indicative of a load condition provided by the drive assembly is also provided. An electronic controller provides a control signal to the shift valve to control the displacement of the hydraulic motor in response at least in part due to the load signal.