[kgzak]

I would use a gyro to make sure that when you want your robot to go straight, that it goes straight and does not turn. Encoders will help you keep your wheels spinning at the same speed which should keep you going straight but if your wheels have different amounts of traction then the gyro can be used to keep the robot going straight. For velocity you can try an accelerometer or just use the encoders, even though it'd be hard to get a good measurement, again because of traction.