A speed control assembly includes an input drive shaft coupled to a first gear subassembly having a rotatable gear, a second gear subassembly coupled to an output drive shaft, and a linkage coupling the first and second gear subassemblies, wherein the input drive shaft, the first and second gear subassemblies, and the linkage are configured such that a rotational speed of the rotatable gear adjusts a ratio of a rotational speed of the output drive shaft to a rotational speed of the output drive shaft. In some embodiments, the first gear subassembly includes a sun gear coupled to the input drive shaft, one or more planet gears, and a ring gear as the rotatable gear. In some embodiments, the second gear subassembly includes a sun gear coupled to the output drive shaft, one or more planet gears, and a fixed ring gear.