Hi, what is your project, what are you doing with the motors?
Increasing the 64kHz will not increase the speed of the motors, and as Mike has said, 255 is full power to them.
A picture of your project and a picture or CAD of your circuit in PDF, jpg or png would be help full.
At the moment you know how your project works and what you want it to do, the forum members don't, so you need to give us as much information, including pictures, so we can help you.
Tom...... 