Abstract: A variable ratio power transmission in the form of a gear box is described, in which the gear ratio of the input and output shafts can be varied continuously between a maximum and a minimum. The power transmission comprises a first bevel wheel (40) rotatably mounted in connection with a U-shaped member (32) on a first axis (34). The U-shaped member (32) is pivotable about the axis (34) and is coupled to the bevel wheel (44) via a sprag clutch (36). Hence, pivoting movement of the U-shaped member (32) in a first direction about the axis (34) results in a torque being applied via the sprag clutch (36) to the bevel wheel (44), whereas pivoting movements of the U-shaped member (32) in the opposite direction has no effect via the sprag clutch (36) on rotation of the bevel wheel (44). A second bevel wheel (50) is also coupled to the U-shaped member (32) via a second sprag clutch (38) which operates in the reverse direction to that of the first sprag clutch (36).