These white plastic wheels have rubber tires measuring 42 mm (1.65″) in diameter and press-fit onto the 3mm D output shafts on many of our motors, including our micro metal gearmotors and our mini plastic gearmotors with 3mm D-shafts. Teeth on the hub optionally allow a reflectance sensor to be used for encoder feedback. These wheels are sold in pairs.
More technical information at Pololu.