I have been using a Polar S520 on my mountain bike. For about a year speed/distance calibration looked quite reasonable. Then about 2 weeks ago it seems like the calibration suddenly changed. Instead of 15 mph speeds I'm showing 25 mph speeds and distance went up accordingly. I checked to make sure the bike setup didn't get changed ( I use it with my road bike too). I checked the calibration number for the wheel/tire size. Both are right where I set them.
Does someone have a suggestion on what else to look for? Is there a battery in the speed sensor that could cause this? Could the magnet to sensor distance cause such a problem and why would that have changed? Everything I can think of would cause the speed to show slower, not faster. Any help would be appreciated.
I'm comparing the speed/distance to both a Cateye Enduro and a GPS track. Cateye shows 10.5 mile loop, GPS shows 9.8 mile loop. Polar use to show 11 mile loop but suddenly changed to 19 miles. I tend to trust the Cateye more than the GPS due to tree coverage over the trail.
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Thread: Polar S520 Speed Sensor Problem