A circuit for controlling an automatic transmission system for an engine includes a resistor for supplying a third signal to the input of a summing amplifier, means for supplying a second signal representative of the speed of the vehicle, to the input of the summing amplifier and a transistor for supplying a first signal to the input of the summing amplifier only when the throttle setting of the vehicle exceeds a predetermined value. The arrangement being such that when the second signal exceeds the third signal or the third and first signals together, the circuit will operate to effect a ratio change to reduce the engine speed for a given road speed and vice versa.