A method and a system for choosing a transmission mode in a vehicle over the course of a road section, in which a simulation of at least one future speed profile vsim for the road section ahead of the vehicle is conducted on the basis of a road slope for the road section. A time period Tappl is determined on the basis of the at least one simulated future speed profile vsim, which ensures that a lower transmission mode is chosen before a lowest permitted speed vmin defined for the road section and/or a set speed vset for a speed control is reached. During the determined time period Tappl, the transmission mode which is utilized when the simulation is made is applicable for the vehicle. After this, it is evaluated whether a lower transmission mode than the currently used transmission mode shall be chosen. A lower transmission mode shall be chosen if the time period Tappl for which the current transmission mode is applicable is shorter than a first threshold period Tlim.