An electronic servo controller, combined with an electric chassis dynamometer and a synchronized mini-computer generated command signal, provides consistent and smooth driving of development vehicles on EPA emission runs. The electronic controller provides the necessary signals to actuate the throttle servo to power the engine, and to generate braking torque on the chassis rolls of the electric dynamometer. The servo gain and compensation circuits allow accurate and stable operation over a wide range of engine loads, transmission gear ratios, and car speeds. A single gain-compensation network is sufficient for vehicles used in AC's catalytic converter development program. Safety shutdown circuits and ease of installation are provided in the design.