These red plastic wheels press-fit onto the 3mm D shafts on many of our motors, including our micro metal gearmotors and our mini plastic gearmotors , to make a nice drive solution for small robots. The wheels have silicone tires and measure 70 mm (2.75") in diameter. Two additional mounting holes for 4-40 screws make it possible to use the wheel with several of our universal mounting hubs. The wheels are sold in pairs.