This paper describes a comprehensive test program for the investigation of the influence of errors and variations in mesh stiffness on the peak stresses developed in spur and helical gears. The study is primarily concerned with speed effects on tooth loads in regions away from resonance. Four sets of specially designed and instrumented gears were tested in an open-loop setup. The interaction between tooth loads and system shaft torques at different operating conditions is also investigated. The results are utilized in the accompanying paper for the development of a generalized dynamic load formula for spur and helical gears.