The synchronizers with locking device, type Subansamble Auto, assures the rapid shifting of the speed steps, safely and noiselessly even under difficult conditions such as vehicle running on a sloping ground.When shifting into an upper speed step, there is no need to double declutch, when shifting into a lower speed step, there is no need to accelerate at the neutral position. This synchronizer facilitates the gear shifting, the vehicle may be dinamically driven and easily adapted to the road conditions and the congested traffic.