Hi, the only thing I can think of is perhaps moving the speed sensor a bit closer to the magnet. You would have to take the motor apart to find the sensor but some older versions had the sensor exposed. Disclaimer: I am not sure if it will make a difference, though...
Check the mechanical integrity and make sure there is nothing wrapped around the motor shaft and that there is no "play", i.e. you can't jiggle the wheel back and forth. I'm sure a mechanical problem can cause a difference.
It's normal for the numbers to vary a small amount, but if the robot is arcing significantly, a difference in speed between the wheels is a possible cause.
Check the front wheels, too, because if one is stiff or not rotating around its post, you can get arcing as well.
Robot Lady
