Originally Posted by pferreira
My Garmin Edge 705 came with a wireless sensor that works on the rear wheel and gives you cadence and speed. The 705 uses GPS first for your speed, but if the GPS loses signal it will use the wheel sensor. It transmits data via a USB cable.
Actually, the 705 works in the reverse, at least that's what I've gathered. It takes the speed from the wheel first, but it does record the location from the sat as well. I have noticed this because on one bike I ride I do not have the speed sensor, and it will "bounce" speed quite a bit more, for instance, 11.5, 12.0, 11.5 all in a few seconds, while I'm pedaling with the same effort, where on my other bike that does have the sensor, as I'm pedaling along and watching my speed it will do 11.5, 11.6, 11.5, 11.7, etc.
Now, the edge 305 did indeed use gps rather than the speed sensor, and it was always prone to the odd jumps, particularly in areas where only a few sats were present (ie, thick tree cover, with a mountain on one side). But id would revert to the speed sensor in tunnels and the like.