Abstract: The present invention relates to an automatic speed gear for bicycles or the like, of the type comprising a pinion assembly, each pinion having a different number of teeth, the pinions being coupled to the hub of a wheel of the bicycle and being adapted to be selectively connected, through a chain, to a driving pinion operated by the bicycle pedals, the switching of the chain from a pinion to another pinion, of the pinion assembly on the wheel, being carried by displacing a chain guide element arranged near the pinion assembly, the speed gear being characterized in that the displacement of the chain guide element, and accordingly the switching of the transmission ratio, is performed automatically, depending on the effort exerted on the pedals and the amount of which can be adjusted depending on the user's requirements.