Algodoo

The problem with this is that you get free energy, they don't behave like gears. If you apply 100 Nm on the red one the blue one outputs 100 Nm too but twice the angular vel

To prove this you can make an engine with it:

Make the blue circle slightly bigger (0.59 radius is fine)
Copy the blue circle and put it between the original blue circle and the red one, then put them slightly overlapped as if they were gears, and hinge the middle blue circle to the background. Then start the scene

That´s why my gearbox never worked

Comment from "Angle doubler": I was trying to make gearbox working like this
(stretched axles with no slip, gear ratio easily changed by script...
but it isn´t as good as i want)