I have a new style, magnetless speed sensor, and lately I've been getting some really erratic readings. Say I'm cruising along on a flat road at 18 mph. The speed on the Garmin will jump up to 21, then down to 17, then 19, settle on 18 for a little bit, and then jump around again. Sometimes it goes totally nuts, and jumps up to like 36. Once or twice, when I was at a dead stop, it showed I was going like 12 mph, until I started moving again. I don't think this happened when I first got the 520, though I can't be sure. Some days are worse than others. I've tried to unpair and repair the sensor, which might help sometimes, for a little while, though again I can't be sure. If I turn off the sensor and rely on GPS for speed, I get smooth, sensible readings. However, I ride in New York City, so GPS readings are often erratic, which is why I got the speed sensor in the first place. I use the Garmin on another bike with an old style speed sensor with magnet, and it doesn't seem to have the same problem.
At first i thought the sensor was wonky, and I called Garmin (after trying to replace the battery, etc.). The sensor was out of warrantee (I got it before I got the 520), but out of the goodness of their heart, Garmin sold me a new sensor for $20. However, when I installed the new sensor, I got the same erratic readings, so the problem apparently isn't the sensor.
Has anyone else seen this happen? I browsed the forum, but didn't see any similar complaints. Does anyone know the cause, or have a solution? I'd appreciate any insight or advice.