Which wheels are suitable for Power Functions M-Motor I'm attempting to build a small self-propelled LEGO robot. Initially I was attempting to use some cheap DC hobby motors but I spent a lot of time trying to attach LEGO wheels to the motor's 3mm spindle and could never get enough torque to make the robot move.

I have now purchased two official Lego M-Motors. Unfortunately I don't have any wheels with a large enough diameter so I'm now looking around to get some matching wheels for the motors. The wheels will need to have a diameter of at least 40mm (the motor's height is 24mm).

I have had a look around but am a bit bamboozled by all the options. Has anyone use one of these motors in a robot and have any suggestions for a wheel type?
The M-motor is directly compatible with almost all the wheels that LEGO has produced, and, with a little building, compatible with every wheel imaginable, even brick-built wheels.

If your problem is that the wheel is smaller than the diameter of the M motor, perhaps you want to attach the wheels to an axle, and then drive the axle with the motor.  There are many different gearing solutions variations that would accomplish this.

If you need to purchase one or more wheels, Bricklink is a great site to buy individual bricks.