I have a Polar RC3GPS heart rate monitor that records, in addition to the heart rate, speed, maps my routes, altitude, target load, etc.

Earlier this month, I accidentally purchased a speed sensor that clips on to the front fork and a small magnet that clips on a spoke. I really meant to purchase the Cadence sensor. I have since added the Cadence sensor and I really like the additional information it provides.

My question is should I return the speed sensor, (it is listed as an accessory for the monitor) or is the a reason to start using the sensor rather than have the speed recorded via GPS.

I don't understand why polar recommends the speed sensor if the gps tracks speed. Thanks in advance.