My wife recently decided she needed to upgrade from a Garmin 200 to a 510. We installed everything and went riding. The speed would switch between whatever we were riding, say 19 mph, to 4 or 5 mph. Caused all sorts of problems with total mileage, average speed, etc. She got a replacement but is still having the same issue. All the tolerances are within the Garmin specs so I'm at a loss. The unit detects the speed and cadence sensor just fine and displays both accordingly but that fluctuation in speed is a mystery. Wheel size, etc. all checks out. Anybody have any ideas? She rides a 51cm Cervelo R3.