a speed governor for a pneumatic power tool which comprises a housing with pressure air supply means and an air turbine including a turbine wheel drivingly coupled to an output spindle and a number of air nozzles for directing motive pressure air onto the turbine wheel , wherein the speed governor comprises a flow controlling valve element associated with an activation piston for adjusting the valve element in response to a control pressure obtained in a pressure sensing opening located opposite an idle running nozzle downstream of the turbine wheel .