[EricH]

It depends... And I say the ideal diameter is 6 inches. (But that's just me.)

The real answer is, it depends. What the terrain is, what the gearing before the wheels is, how many wheels there are, and what type of wheels/orientation are all going to be big factors.

4" wheels will need less gear reduction off the gearbox for the same speed, and are lighter, in general. However, the tradeoff is ground clearance and climbing ability (2" obstacle max without some sort of assistance, which could be a 6WD drop center rocking back). I'm not saying that a 4" wheel can't climb, mind you--however, placement and number of wheels need to be considered before evaluating climbing ability.

8" wheels need more gear reduction for the same speed and will weigh more than a 4" wheel. However, they increase the ground clearance and climbing ability--at the expense of a low CG. Again, this can be designed around.

6" wheels are in the middle in terms of gear reduction and weight and climbing ability and ground clearance. They're better than a 4" at ground clearance, and better than a 8" at weight and gear reduction.

The real question is, what does the game require?