Test results with a high speed four square gear test rig (nmax = 26 000 rpm) show the influence of speed@ viscosity@ base oil@ type and amount of additive on the scuffing load. In some cases at high speed the scuffing load can be more than twice the value calculated according to ISO/DIS 6336/4. The reason for the speed-dependency of the scuffing load is explained by the kinetics of the chemical reaction between the metal and the additive. It is described@ how a calculation method could take account of the high scuffing load at high speed.