A propulsion system for a vehicle has output shaft (2) of a combustion engine (1), which is connected to a first component (9) of a planetary gear so that they rotate at a first speed (n1). An input shaft (3) of a gearbox (4) is connected to a second component (11) of the planetary gear so that they rotate at a second speed (n2). An electrical machine is connected to a third component (10) of the planetary gear so that the third component (10) rotates at a third speed (n3). A control unit (17) estimates in appropriate operating situations a desired speed (n2) for the gearbox input shaft (3). The unit receives information about the speed (n1) of the engine output shaft (2). It causes the electrical machine to give the third component (10) of the planetary gear a speed (n3) which in conjunction with the speed (n1) of the engine output shaft (2) results in the gearbox input shaft (3) assuming the desired speed (n2).