After some general considerations about the extended use of Continuously Variable Transmissions (CVT's) for lightweight vehicles, which clearly show the two-fold advantage, the first of a very simple, compact and well integrated with engine transmission, the second to make possible the calibration of the engine only in a limited operating range, the efficiency of this type of transmission is investigated.A theoretical analysis of CVTs is carried out, in order to identify all the characterizing physical parameters. The design of a specific test rig is then presented. Since the transmission ratio depends on forces equilibrium, the centerline force appears to be the most important characterizing parameter. The new system is particularly conceived for small power output, i.e. minimum friction losses.The experimental results carried out on different elastomeric belts clearly show the dramatic influence on transmission efficiency of the axial force modulation, particularly in low transmitted torque and On the proposed test rig, with minor changes, it is possible to test actual transmission components as in real operating conditions.