Driving a manual transmission ?

A manual transmission is directly connected to the motor through the clutch, thereby the speed you are going and gear you are in will directly affect the RPM of the motor. An automatic is connected to the motor through the torque converter, this uses hydraulic force to turn the transmission until you get to a certain speed an overdrive than it locks up. When you take your foot off the gas it unlocks and works on the hydraulic area again, it wont downshift until your speed lowers to a certain level, or you press hard on the accelerator. In unlocked mode there is usually a 30% slippage between the speed of the engine and the speed of the impute shaft. A certain amount of power is used to operate the automatic transmission, whereas the manual transmission uses very little effort to turn, thereby you get more power to your wheels. As far as RPM and efficiency, that goes more engine load, a engine with very little load in its prime operating range will be more efficient than a motor that is over loaded. To illustrate this I will use a couple of minivans, 1995 olds silhouette will operate at 1600 rpm at 65 mph and get about 25 mpg, a 1994 ford arowstar will operate at 2200 rpm at 65 and can get 28 mpg, I have owned both these vans and use this reference because of similar weight, and aerodynamics, yet different tuning and gearing.