A secondary controller for controlling the performance of a moving automobile is described. The secondary controller can be configured to communicate with one or more vehicle controllers, such as the engine control unit, while the automobile is being driven. The secondary controller can send control commands to the vehicle controller to effect the operation of the vehicle's power train. The secondary controller can receive power train related data from the engine control unit and based upon the received power train data determine when to send the control commands. In one embodiment, the secondary controller communicates with the vehicle controller via the vehicle's diagnostic port, such as an OBD-II port. In another embodiment, the secondary controller can be configured to control the operational displacement of a variable displacement engine while the vehicle is driven.