A reduction group for a vehicle and a reduction assembly including the reduction group are provided. The reduction group has a ring gear, a central sun gear, a planetary gear holder and at least one planetary gear on the planetary gear holder. The at least one planetary gear has a gear meshed externally with the ring gear and internally with the central sun gear, a planetary gear shaft fixed to the planetary gear holder and a pair of bearings supporting the gear on the planetary gear shaft. The planetary gear shaft and the planetary gear holder are engaged so that the planetary gear shaft is axially adjustable on the planetary gear holder in an operational configuration in which the planetary gear shaft performs an axial tightening action on the pair of bearings exhibiting an axial clearance adapted to allow rotation of the pair of bearings.