Originally Posted by Steve B.

Thanks for the info., John but a question.

Does anybody know for sure how a Garmin 500-520, 800 - 1000 series device, that is providing on-screen mapping (thus needs GPS) as well as speed, actually works ?.

If it's providing an on-screen map, it's going to be doing all the GPS data collection to generate position and thus mapping, as well as and as a side effect provide speed.

I would suspect that the software will use the speed sensor data as the "better" data, in place of the speed data coming from the real time calculations of GPS "every one second" breadcrumb style tracking. But I also suspect that the GPS speed tracking is going to occur even with the presence of a speed sensor, if only in the event the speed sensor sends erroneous data or craps entirely.

Thus I suspect there's no saving of battery life by using a speed sensor as I suspect the unit is GPS data processing for speed at the same time, just for giggles.