A method for controlling a vehicle's speed includes adopting a desired speed vset for the vehicle; determining by means of map data and location data a horizon for the intended itinerary which is made up of route segments with at least one characteristic for each segment; 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: making a first prediction of the vehicle's speed vpred—cc along the horizon with conventional cruise control when vset is presented as reference speed, which prediction depends on the characteristics of said segment; comparing the predicted vehicle speed vpred—cc with vmin and vmax, which demarcate a range within which the vehicle's speed is intended to be; making a second prediction of the vehicle's speed vpred—Tnew along the horizon when the vehicle's engine torque T is a value which depends on the result of said comparison in the latest preceding simulation cycle (s−1); determining at least one reference value for how the vehicle's speed is to be influenced on the basis of at least one of said comparisons in that simulation cycle (s) and the predicted vehicle speed vpred—Tnew; sending to a control system in the vehicle said at least one reference value with respect to which the vehicle is thereupon regulated. The invention comprises also a module for controlling a vehicle's speed.