Two Idlers, Two Cogs, and 40 Tread Links - and a pair of GM2/3/8/9 style gear motors. A treaded vehicle is just waiting for this package!
This long-motored 360:1 Mini Metal Gear Motor is a torquey, compact brute!
Looking for a good continuous-rotation servo? The Grand-Wing Servo GSV0008A is a well proven unit!