One of the things that our team noticed last year is that that one side would move faster than the others because of the way in which the CIMS are oriented. Inside your motors are wire windings, depending on which direction they are wound gives a mechanical bias in one direction.. Typically on a tank drive your motors are in oriented on the opposite side of the robot, which can cause the motors to have reverse winding directions...
Best way to fix this would be to regulate down the voltage on one side. Hope this helps
