In the first chapter it determines precisely, the dynamics-efficiency, but at the gears transmissions, the dynamics efficiency is the same like the mechanical efficiency; this is a greater advantage of the gears transmissions. This advantage, specifically of the gear's mechanisms, may be found at the cam mechanisms with plate followers as well. In the second chapter one presents shortly an original method to obtain the efficiency of the geared transmissions in function of the contact ratio. With the presented relations it can make the dynamic synthesis of the geared transmissions having in view increasing the efficiency of gearing mechanisms in work. We calculate the efficiency of a geared transmission, having in view the fact that at one moment there are several couples of teeth in contact, and not just one. The start model has got four pairs of teeth in contact (4 couples) concomitantly. The best efficiency can be obtained with the internal gearing when the drive wheel 1 is the ring; the minimum efficiency will be obtained when the drive wheel 1 of the internal gearing has external teeth. For the external gearing, the best efficiency is obtained when the bigger wheel is the drive wheel; when we decrease the normal angle alpha0, the contact ratio increases and the efficiency increases as well. The efficiency increases too, when the number of teeth of the drive wheel 1 increases (when z1 increases). Nearly all the models studied the dynamic on gearing with axes parallel, is based on mechanical models of classical (known) who is studying spinning vibration of shafts gears and determine their own beats and strains of shafts spinning; sure that they are very useful, but are not actually join formed of the two teeth in contact (or more pairs of teeth in contact), that is not treated physiology of the mechanism itself with toothed gears for a view that the phenomena are dynamic taking place in top gear flat; model [1] just try this so, but the whole theory is base