Abstract: In a stepless transmission gear system for automobiles, a variable-diameter pinion mechanism cooperates with a stationary internal gear to permit the speed ratio of the input to output shafts thereof to be changed steplessly from a substantially infinitestimal ratio to an overdrive ratio, the diameter of said pinion mechanism being continuously variable with the eccentricity thereof off the axis of the input shaft. Cars employing the present transmission gear system could dispense with a clutch and even a brake.