Monkey's response of the 19th makes a lot of sense to me. Cruise control will attempt to maintain a constant speed regardless of whether the road is level or on a gradient. Consequently, it will be applying more accelerator when going uphill. A driver who is trying to minimise fuel consumption will, however, anticipate the situation and make better use of momentum before approaching the hill and then ease off the accelerator slightly as the car is going uphill. This is something cruise control cannot do but, as Craig says, it can still take fatigue out of driving.