Under load, it's calculated to be around 550rpm. When put to a 4 inch wheel, top speed is calculated at 9.67. I'm assuming 2 inch wide roughtop wheels for this system, but this gearbox could be mated to quite a few different systems.

By the way, if anyone wants the CAD for this, feel free to PM me. It wasn't too difficult to make, so I'd have no problems handing the CAD out.