Changing Speed data of Motion Script

Hey,

I have a car and i inserted Motion script to it. As you know Motion Script have some parameters such as speed Look at speed and etc. I want to change this speed data by using my own script. how can i do ?

I tryed this.actor.Motion.Speed but it returns unidefined value. Can you help ? I also want to show some parameter in the Proporties panel.

Thank you in advance.