Hi when it comes to big wheel robots perhaps I may be able to help, The Saint has 600mm wheels but with the tyres they are 670mm diameter the Mighty Seraph (Feather weight robot) has a wheel diameter of 500mm.

Material used UHMWPE or the green stuff from Plastics Direct Bolt the tyre on I used bike tyres for both robots.

The transmission, the rule of thumb would be double the diameter of the wheel, half the speed of the wheel, to get the same speed for the robot.

So if you connected a sprocket to the drill motor say 12 tooth sprocket, and the wheel had a 24 tooth sprocket, the speed of the 200mm diameter wheel robot would be the same as an equivalent 100mm diameter wheel robot.

Hope this helps