Are turn rate indicators calibrated to a certain speed? If so, what speed?
They are not calibrated for a certain speed.

If by "calibrated", you mean they deflect together / to the same side, then you're using the instruments wrong. The two instruments have are not linked to each other, they operate independently and they operate on different principles. They are located next to each other in the cockpit out of convenience.

The "slip indicator" is a ball in a U-shape tube. It tells you whether gravity (or experienced acceleration) is pointed down, or to the side. Period. It is as simple as that.

The "turn coordinator" (little white plane / white needle) indicates your yaw rate. It operates by using a spinning gyro; the precession effect of yawing causes the gyro assembly to roll.



The roll is restricted by a spring, which is calibrated to indicate standard rate turn when the yaw rate is 3 degrees per second. The turn coordinator works at any airspeed (part of the preflight check is to turn the plane while taxing and verify the turn coordinator works).