When I was a kid, I came up with this one.
I wanted to make a go cart using an old lawn mower engine.
The problem then, was the motor was vertical, while the wheels were horizontal.
Then answer: A Ball on a shaft connected to the wheels.
The ball rolls on a circular plate that is driven by the motor.
The ball can be positioned from the middle of the plate the outside of the plate which makes it turn faster.
That was a simple design
From a stupid little kid (me).
A friend of mine told me they have variable ration transmissions.
They use a piston with a variable stroke length to feed another piston driving the wheels.