Abstract
In many experiments it is only possible to load a molecular motor by manipulating the viscous drag from the fluid medium. In these situations, it is not clear how to measure the efficiency of energy transduction. Here we define the Stokes efficiency as the average viscous drag times the average velocity divided by the rate of free-energy consumption. We show that this efficiency is bounded by 100% and thus is well defined. We compare the Stokes efficiency with the usual thermodynamic efficiency, and discuss what information it provides about the motor mechanism.