A gearbox with a transmission input shaft and an output shaft mounted parallel in a gearbox housing and includes several pairs of engaged pinions to shift in a corresponding number of forward transmission ratios. A reverse gear axle is mounted in the housing parallel to the input and output shafts, with two idler gears freely rotatable on the shaft, one gear engaging a fixed gear on the input shaft, the other engaging a fixed gear on the output shaft. A reverse synchronizing device is located between the two idler gears for engaging reverse gear ratio. The synchronizer includes a jaw clutching sleeve splined to one of the idler gears, and is slidable towards the other idler gear to actuate a synchronizing clutch. The clutch has radially inward facing champfered blockers which engage corresponding slots in the sleeve. A synchonization initiation device in the form of a snap ring is also provided.