Normally , when you maintain the same speed by using your feet , you will be more consuming then by the CC . Why ??? because the cruise control just keeps the same speed . You just cannot keep your feet just as stable as the CC can . Even let's say 1/100 mm is a change in fuel to the engine , and you do that maybe every 1/10 sec , without you knowing it off course . If however you just want to use less , than the feet can keep +/- the same throttle position and the speed will go down when driving , uphill , windwards ... . The CC will then use more because it keeps the speed constant and adjust the throttle to maintain that .