Most of the judges will give you a simple choice when you plead, guilty or not guilty they don't want to here guilty with reason, or anything like that, except guilty by reason of insanity. Some may allow a reason for the infraction if there were a legitimate reason such as driving a big rig past a 6,000 pound limit sign when the location you are going to is a direct line to the road in which you are travelling. As to a speedo error or calibration being off due to larger tires or a defect usually won't fly.