A versatile, variable gear ratio mechanical transmission system is providing having similar first and second spaced Disc Subsystems connected by a belt or chain. Each subsystem of which comprises three primary components: (1) a Shift Actuation Structure located at one outermost side; (2) a Receiving Disc located at the opposite side; and, (3) a Retention Disc sandwiched between. The Retention and Receiving Discs accommodate concentric rings of proportionally spaced holes in which corresponding smooth, contoured or toothed Headed Pins are maintained, engaged, or retracted. When the pins are engaged, they span the slot between the Receiving Disc and Retention Disc and interface with the conveyance transferring load from Driving Disc Subassembly to the Driven Disc Subassembly.