" It will only show the proper rate at one airspeed though, as DFC stated."
This is really not correct. A properly operating turn coordinator directly measures *rate of turn* through gyroscopic techniques. It is independent of airspeed. If you have the aircraft stationary on the ground, put it on a turntable and turn it at standard rate, 3 deg/sec, the turn coordinator (or turn and slip) will show a Rate 1 turn.