Use a encoder in each wheel to calculate distance traveled and use a electronic compass to tell you the direction. Encoder are not really accurate to compute the orientation. You need a microprocessor to do the calculations. To know how to do the calculations make a Yahoo search for “Dead Reckoning” in the context of Robotics. You can get all from: http://www.lynxmotion.com http://www.robot-electronics.co.uk http://www.acroname.com http://www.parallax.com http://www.robotshop.ca http://www.pololu.com If you get the Lego Mindstorms NXT you can do this and many more projects easily.