[Schroeder]

Good question.
Do both indicators show the same speed? If not, one might be your true airspeed indicator.
The higher you fly the less speed is indicated on your gauges so that you get quiet a discrepancy between indicated and true airspeed if you fly high. Some planes have a secondary gauge that shows the true airspeed (or at least it is supposed to show the true airspeed, I'm not sure how accurate this is as
I never use them).