[Tom I]

Well I'm assuming he'd have all 3 wheels on the same level, as in the axles would be at different heights off the ground for the 6" and 8" wheels, so that all wheels touch the ground.

I don't think you'd have a torque difference, man. Think about it, in order to have the wheels spin at the same relative speed in terms of ground distance, you'll need to increase the rotational speed of the 6" wheels. Thus, if you increase the speed, you'll decrease the torque. So the 8" wheel axle will have more torque than the 6" wheel axle, but when you then consider the 8" wheel will cover ~25" of ground distance per rotation as opposed to the 6" wheels which cover ~18.8," the wheels will end up having the same relative torque because they'll have the same relative speed. Using the same drive source, if you increase power, the inverse happens to torque, and vice versa, so all in all it comes out being equal.