At least one reference value is determined for controlling a vehicle's speed by a method that includes predicting, in a first prediction, a first speed based an engine torque that retards the vehicle as compared with a conventional cruise control along a horizon, and, in a second prediction, a second speed based on an engine torque that accelerates the vehicle as compared with the conventional cruise control; comparing the first and second speeds with a lower limit value and/or an upper limit value which delineate a speed range within which speed is maintained, with an offset added to said lower and/or upper limit values if the vehicle is in a route segment that includes a steep hill; and determining the at least one reference value based on the respective comparisons and the first and second predicted speeds.