A transmission control system can include a main transmission. The transmission control system can also include a sub transmission that has two forward transmission gears mechanically shiftable in response to a shift operation. The transmission control system can further include a controller that shifts the transmission gears of the main transmission by activating a shift control motor, and control the clutches configured to enable and disable transmission of a rotational driving force of an engine. When one forward transmission gear of the sub transmission is shifted to the other forward transmission gear in response to the shift operation, the controller can relax engagement of the clutch if an engine speed exceeds a clutch release speed.