A transmission speed control system for a walk-behind lawn mower includes a transmission, a transmission positioning assembly, and an electronic control unit. The transmission is tiltable about and operably engaged with a drive axle shaft. The transmission positioning assembly tilts the transmission between first and second angular positions. A belt tension between a drive belt and a transmission pulley of the transmission continuously increases from a minimum belt tension when the transmission is in the first angular position to a maximum belt tension when the transmission is in the second angular position. The rotational drive force exerted on the drive axle shaft by the transmission and a resulting driving speed of the lawn mower vary proportionally with the belt tension. The electronic control unit controls the transmission positioning assembly to tilt the transmission based on target and detected driving speeds.
