BrickPi.MotorSpeed will not work for the BrickPi3. Since that command does do something we can safely assume it is not the BrickPi3.
@vlvl5 can you explain what you mean by 200m/s? When you set the motor speed you are adjusting the Pulse Width Modulation, PWM. This pulses the motors really fast to adjust its speed. In order to read the motor's speed you would have to measure at different times how much the wheel turned. For example the wheel starts at 0 degrees and 0 seconds have gone by, then 1 second later the motor is at 360 degrees. That would mean the motor is turning at 1 rotation (360 degrees) per second. The BrickPi3 can do this automatically but the older BrickPi's can't, so you'll have to code it.