A method for controlling a vehicle's speed by adopting a desired speed vset for the vehicle; determining a horizon for the intended itinerary made up of route segments, effecting the following during each of a number of simulation cycles (s) each comprising a number N of simulation steps conducted at a predetermined frequency f: first predicting the vehicle's speed vpred—cc along the horizon with conventional cruise control when vset is presented as reference speed, comparing the predicted vehicle speed vpred—cc in a range vmin to vmax, making a second prediction of the vehicle's speed vpred—Tnew along the horizon based on the vehicle's engine torque T; determining a reference value for how the vehicle's speed is to be influenced on the basis of at least one of the comparisons and the predicted vehicle speed vpred—Tnew; controlling the vehicle by the reference value. A module for the foregoing controls a vehicle's speed.