Traditional Auto's use more fuel because of the ratio's they use. Eg, In manuals you have 5-6 different speeds these days allowing you to better pick the right gear for the conditions, older auto's like that in the Corolla have only 3-4 speeds, so immediately the car has to make do with less and is revving higher in lower gears to reach the same speeds. Auto's also have a tendency to absorb more torque from the engine, so more acceleration is required to get to the same speed as a manual. However, New Generation Auto's with CVT or more gears (Mercedes' 7 speed is a beaut!) are now proving to be just as efficient as manuals because they have a greater spread of ratio's.