PURPOSE: To improve fuel consumption by repeating high speed revolution and idling of an engine alternatively and by driving a car in an inertia running mode, using an overrunning clutch at the time of the idling.

CONSTITUTION: A car is brought to a fixed speed driving state from a stationary state by usual procedures. While the fixed speed driving, at 80 km/hour for instance, an engine 1 revolves at 2,000W2,500 rpm. Then, the accelerator depression is removed, an overrunning clutch is off and the engine 1 becomes idling revolution (500W800 rpm.) The car, after running for a while at the fixed speed, becomes slow effected by air resistance, etc. When the speed drops less than the fixed level, 65W70 km/h for instance, depress the accelerator again to increase the engine revolution to 2,000W2,500 rpm and the overrunning clutch is on again to accelerate the car speed up to 80 km/hour.