If you are seeing unusual speed and distance from your Wheel Sensor, first confirm that your wheel circumference is set correctly for your bike in Garage > Bike Details > Wheel Sensor settings.
On Hub Dynamo wheels
In some cases, a dynamo hub (special kind of wheel hub that generates electricity from motion to power lights) can interfere with a Wheel Sensor mounted in proximity. The Wheel Sensor uses a magnetometer to detect rotation and the magnetic fields generated by a hub dynamo can interfere. This often results in the Wheel Sensor showing unusually high speed and distance.
In these cases, we suggest moving the Wheel Sensor to the rear wheel. This usually solves the problem.
More details on how the sensor operates can be found on Garmin's support center.