Hello @spycatcher2k,
first of all thank you answering. I am not sure if I am driving it with PWM, I suppose yes i do. My motor shield is mounted in my arduino Mega so I guess so. I do not really want to measure the Voltage. I would like to have same RPM in all of my wheels when moving forward. It seems that one or two of them are turning with different RPM and as a result the vehicle seems to have a little drag to the right or left depending to the wheel with the lower RPM.
I hope this is clear enough,