1. A method performed by a network node for estimating a relative tire friction performance of a number of vehicles, wherein the network node and at least one communication unit located in each vehicle are comprised in a communications network, the method comprising: receiving continuously tire-to-road friction values and corresponding Vehicle State Parameters (VSPs) from each communication unit in the number of vehicles during driving thereof, each VSP comprising at least a geographical location at which a tire-to-road friction value is measured and an instant in time when the tire-to-road friction value is measured; determining that a tire-to-road friction value of a vehicle in the number of vehicles is comparative to at least one other tire-to-road friction value of another vehicle in the number of vehicles, when the corresponding VSP of said at least one other tire-to-road friction value deviates less than a predetermined deviation range from the corresponding VSP of the tire-to-road friction value of the vehicle; comparing the tire-to-road friction value of the vehicle to the at least one other tire-to-road friction value of the another vehicle; and determining a ranking of a relative tire friction performance of the vehicle in relation to at least the another vehicle based on said comparison.