Direct drive minibot - output diameter?

[Ether]

1) You have to check that the torque is available to accelerate the mass of the machine from a standing start at the chosen drive ratio (diameter). Otherwise your bot just sits on the launch pad and smokes. This calculation should be fairly easy, though.

The highest torque output is at stall, not at max power. So if you have enough torque to maintain a speed of V at max power, the torque only gets higher as the speed decreases. So it's not a question of enough torque. It's a question of will the motor overheat. And you can't calculate that from the published motor data.

2) The rate of acceleration must be taken into consideration if you want to (and you should) optimize elapsed time rather than simply a top speed there may not be time/distance to reach. Improved acceleration from a smaller shaft trades off against reductions in top speed. This is a somewhat harder problem, but certainly solvable

You read my mind. It's on my to-do list. Just haven't gotten to it yet. I suspect however that the accuracy of such a calculation will be driven largely by unknown factors like friction, motor tolerances, tire slip ratio (especially with compliant material such as surgical tubing), battery condition, etc.

As a practical matter other factors argue for a far more conservative value than this optimization would give anyway.

Other factors like those listed above, or did you have others in mind?