Servo Wheel

via Adafruit
SKU AF-167

Plastic wheel with a cutout specially designed to allow attachment to our larger continuous rotation servo. Makes it easy to get your robot rolling. Each wheel comes with a rubber tire that is quite sturdy. To attach, just remove the horn from your servo, align the splined gear, and use the small screw that came with your servo to attach permanently

You'll probably want to get 2 or more.

PDF with wheel dimensions