Disclosed is a technique for inducing economic driving for a vehicle. In particular, a control unit calculates a driving resistance of a vehicle based on a current rolling friction, air resistance, and a road grade of the vehicle. The an amount of air-conditioning energy consumed by the vehicle is calculated by the control unit, and in response the calculated driving resistance and air-conditioning energy are substituted into a data map to continuously calculate the optimal economic vehicle speed of the vehicle in real time. The optimal economic vehicle speed calculated by the control unit is then displayed to a driver via a display unit.