Join Date: Apr 2010
Location: Fredericton, NB, Canada
Bikes: 2010 S1, 2011 F75X
According to Garmin, the Speed Sensor is the primary source if the Edge detects it. That said, have a look over in the Garmin Forums - there may be a glitch in the latest firmware that makes it perform the opposite.
For example, there's a guy who stores his bike in an underground parking garage. When he starts his ride from inside that garage, his 500 will measure speed, but it won't measure distance until he's outside and gets a signal. But if he turns the GPS off in the menu, he gets proper speed and distance.
I have the 500 as well with the speed sensor. I have yet to test this theory - but so far, the unit has worked flawlessly. I get really good coverage in my area. The first time I turned on the 500 in my basement, I got full reception in under 10 seconds.