Originally Posted by chasmm

The GSC-10 speed/cadence sensor is an accessory that mounts on the left rear chainstay and picks up signals from a magnet mounted to the crank (for cadence) and the rear wheel (for speed). Wait you say...the 500 (or any other Garmin cycle computer) already has GPS...why does it need to use the wheel revolutions to measure speed?

The reason is that while over a long distance, GPS is very accurate, for short stints, it MAY not be. Thus the sensor to measure wheel revolutions.

The logic on the 705 and 500 is supposed to be always use the GSC-10 for speed, and if for some reason it can't use the speed/cadence sensor, then it should default to GPS. Someone posted about a bug on the 705 introduced in the last firmware update (which has subsequently been pulled) that mucked up that logic. We (meaning myself and many other 500 users on the Garmin support forums) that the same bug is present in the most recent 500 firmware also. Many of us have seen the speed number fluctuate while riding under cover such as an overpass, trees, tunnels, parking garage, etc. This should not be happening. That it does indicates that the 500 is having problems calculation speed when the GPS loses signal, and that shouldn't be a problem if the GSC-10 is present.