Abstract: A hydromechanical transmission has an engine-driven power shaft connected to a variable-volume pump forming the input side of a hydrostatic unit whose output side comprises a constant-volume motor connected to this pump. A summing planetary-gear train has a first ring gear connected directly to the power shaft, a second ring gear connected to the hydraulic motor and a planet carrier with two interengaging sets of planet gears, one set meshing with the first ring gear and the other set meshing with the second ring gear as well as with a sun gear. An output shaft of the transmission can be connected via a high-range clutch to the sun gear of the summing train or via a low-range clutch to the planet carrier thereof to provide a speed ratio which is steplessly variable in two ranges.