High quality wheels known firm Pololu.
Servo DGS S04NF operates as a variable speed motor. It is supplied with the voltage from 4.8 V to 6 V, the torque of 3.5 kg*cm, speed 0.15 sec/60. Complete with compatible wheel with a diameter of 38 mm.
Set includes two DC motor powered with the voltage from 3 - 6 Volts along with the adjusted wheels with a diameter of 65 mm. The set also includes magnetic encoders with a resolution of 8 impulses per revolution.