Change voltage to Motor

Is there a way through C to vary the voltage going to the Cim motors. This would then vary the speed. Or do we have to do a sprocket change to slow down the RPM of the wheels?

Yes you can vary the speed of a motor through C if you are using a Victor Speed Controller. By changing the PWM output in C, to the Victor controlling that motor, you will vary the speed of that motor. The Victor Speed Controller allows you to control a motor from full reverse to full forward, or anywhere any between.