Pololu 75:1 Metal Gearmotor 25Dx69L mm HP 12V with 48 CPR Encoder 22 kg⋅cm

These cylindrical brushed DC gearmotors are available in a wide range of gear ratios and with five different motors (two power levels of 6 V motors and three power levels of 12 V motors). The gearmotors all have the same 25 mm diameter case and 4 mm diameter gearbox output shaft, so it is generally easy to swap one version for another if your design requirements change (though the length of the gearbox tends to increase with the gear ratio). The motor and encoder portion is available by itself (no gearbox) for each combination of power level and nominal operating voltage, and versions without the encoder are also available.