  The thermodynamic uncertainty relation provides an inequality relating any
mean current, the associated dispersion and the entropy production rate for
arbitrary non-equilibrium steady states. Applying it here to a general model of
a molecular motor running against an external force or torque, we show that the
thermodynamic efficiency of such motors is universally bounded by an expression
involving only experimentally accessible quantities. For motors pulling cargo
through a viscous fluid, a universal bound for the corresponding Stokes
efficiency follows as a variant. A similar result holds if mechanical force is
used to synthesize molecules of high chemical potential. Crucially, no
knowledge of the detailed underlying mechano-chemical mechanism is required for
applying these bounds.
