A method and device for driving assistance for a propulsion unit of a vehicle with manual control of the transmission gear ratio, using a control system with a calculator controlling an indication to a driver of a recommended gear ratio based on a request information from the driver and depending on driving conditions of the vehicle at any moment, and a mapping of the gear change rules from which the ratio to be recommended is determined, depending on the vehicle speed at each moment, for an optimal fuel consumption and a minimized pollutant emission. The recommended ratio for an optimal consumption is corrected in successive steps by taking into account, one after the other, a set of constraints concerning the driving safety and comfort, each step having priority over the previous one so that the ratio finally recommended is an optimal balance between the different constraints while putting an emphasis on driving safety.