A device for controlling an automated transmission with discrete gear ratios of a motor vehicle power plant able to preposition at least one transmission system operating parameter as a function of the next gear change to be performed, comprises a means (1) of estimating the gradient (P) of the road surface along which the vehicle is running, and comprises a stored map (9) linking the accelerator instruction, the gradient (P) of the road surface, and the sign of the actual acceleration of the vehicle. The device comprises a logic processor (3) which continuously delivers a Boolean upshift variable signalling a forthcoming upshift or downshift in transmission ratio, as a function of which Boolean variable the said operating parameter is prepositioned.