On a recent flight (fixed-wing aircraft) I noticed that the difference between the airspeed and ground speed was as much as 20+ mph when at the same time the indicated wind speed was just under 7 mph. How can this be and is this most likely and issue with the airspeed sensor, GPS, or some other reason I am overlooking?
Did you followed the instructions Calibrating an Airspeed Sensor ?
Important part: After you start up APM on your aircraft you should wait at least 1 minute for your electronics to warm up, preferably longer, and then do a pre-flight calibration of your airspeed sensor.
Sensor is drifting quite allot if it heats up, so maybe worth to check if it improves your airspeed accuracy.