A fuel-efficient automotive vehicle is provided having a power train including a primary internal combustion engine, an auxiliary engine, and a coupling system which selectively transfers power from the auxiliary engine to the primary engine when the speed of operation of the auxiliary engine equals the speed of the primary engine. The speeds of both engines are controlled by separate gas pedals conventionally located within the vehicle. The primary engine is of smaller power and better fuel efficiency than an engine which would generally be required by the vehicle. Although the primary engine can maintain the vehicle at a cruising speed, it relies upon the added power of the auxiliary engine for acceleration and hill-climbing.