In a vehicle having an all-wheel drive system in which the front axle and the rear axle of the vehicle can be driven with differently sized front-axle and rear-axle torques, a rotational-speed difference between a front-axle-side rotational speed and a rear-axle-side rotational speed is established from a torque difference between the front-axle and rear-axle torques. The vehicle includes a unit for determining a road friction coefficient, which unit measures the rotational-speed difference and determines the road friction coefficient based on a value pair derived from the rotational-speed difference and the torque difference or determined from a parameter correlating therewith.