I put a summary referring to a type of transmission and hydraulic speed change that I already exposed in 1998.
Instead of the law of the lever, a principle used by gearboxes, here a liquid circuit is used in which the resulting difference in speed and power is achieved by regulating the amount of flow circulating through the circuit while maintaining the same power and revolutions of the engine that drives it.
Thus, for example, if the motor rotates at 3000 revolutions per minute, moving also to 3000 r.p.m its motor gear pump, and then we regulate the flow that we supply to this gear pump, with this, we also regulate the speed of the outgoing flow toward the transmission or impulse pump, and therefore the transmission speed and output power of that pump.
The result is that:
With the same power and revolutions of the motor (3000 r.p.m.) we can chose more flow of the fluid y speed of the car, o less flow of fluid and more power in the motion of the car.
In this device there is only a variable speed from 0 to what we want to expand, being able to also add to the device, brake and clutch systems if necessary.

This hydraulic transmission can be partial or simple hydraulic speed change, where both pumps, (the motor pump, and the transmission pump) can be together, with which the transmission to the wheels will be done as tridicionally with axles and mechanical ball joints.
Or it can be complete, in which case the transmission from the motor pump is made by a flexible fluid circuit, and the trasmission pumps will be located on the wheels, and therefore it does not need transmission of rigid axles and ball joints.