Based on gear scuffing results obtained on a high speed gear test rig, equations for calculating bulk temperature as well as critical temperature of a lubricant/steel combination are presented for dip lubrication. The calculation for bulk temperature of gears is modified to include speed as well as applied load. Critical scuffing temperature, previously thought to be independent of operating conditions, is found to be greatly influenced by gear speed and oil type. Experiments show that synthetic oils increase the value of critical temperature dramatically in comparison with base mineral oils, and speed also alters the critical temperature. The curves of critical temperature vs. pitch line speed of gears are characterized by a parabolic shape which is verified experimentally and mathematically. The effect of speed is attributed to a chemical process in which the loss of reaction film is related with the length of time in the contact zone. The effect of oil type is explained by the strength and concentration of reaction films on the contacting surfaces. Finally, a modified equation is presented for converting scuffing load limit between simulated and practically used gears.