Abstract: An adjustable rotational transmission assembly 10 coupled to and provides adjustment of final drive ratio for a driven device 12, such as a bicycle, moped, motorcycle, vehicle, etc. Transmission assembly 10 adjusts rotation or drive radius and drive circumference to adjust final drive ratio to the driven device 12. Transmission assembly 10 provides infinitely variable drive ratios within a range by adjusting the size of the drive circumference path for movement of drive cable/chain ends. Assembly 10 allows shifting or adjusting of drive ratio with or without requiring any drive rotation of assembly 10. A first example embodiment 14 of the present invention assembly 10 has adjustable transmission crank arms 22. Crank arms 22 are adjusted to retracted and extended positions.