Originally Posted by
njkayaker
If the speed sensor is present (and working), the Garmin 800 uses that in place of the GPS-determined speed. (Presumably, the 500 works the same way.)
The only caveat I'll add here is you're much better off if you manually enter your wheel size in your Garmin settings rather than going with "auto calculate." I've found the mileage can be substantially off if you use auto calculate and if your wheel size is being interpreted incorrectly, it will impact your speed too.