A navigation system of a motor vehicle has an input unit, a position-determining unit, a central processor unit (CPU) for calculating a route between a first and a second location by reference to map data which contains information for determining a predicted fuel consumption for the route, and an output unit for outputting travel instructions which are matched with the current position of the vehicle. A maximum travel time for a journey from the first location to the second location is predefined by a user using the input unit and the route is determined such that the predefined maximum travel time is not exceeded, and the anticipated fuel consumption is minimized.