[I]Automatic transmissions take a little bit of power to operate. They use a small amount of electrical power, they have a hydraulic pump which holds the clutches/bands in place. This takes a little bit of engine power to work. Also, there is a little bit of efficiency loss in the torque converter as well because it allows a difference between engine speed and transmission input shaft speed.
Because of this a manual transmission will get better mileage. The torque converter on an automatic multiplies torque when it is not locked. This is why automatic transmissions use different gear ratios. Because there is torque multiplication in the torque converter they have to use slightly lower ratios to maintain the desired economy/performance balance. However, a torque converter can lock during steady state cruising. When that happens the lower ratio means that the engine turns at a lower RPM than the manual transmission at similar speeds. This can allow for a slight advantage in highway MPG if the transmission is efficient enough in other areas.
The only thing this doesn't account for is the human component of the manual transmission. A person can anticipate a turn or a period of acceleration. Also, a person can change their shifting style at any time to accomodate their driving.[/I]