Want to get a 510 but just wanted to make sure it does what I want it to.

On the sales video it shows you riders able to tell where other riders are in relation to themselves on the road realtime – ie rider a is mile ahead, rider c is 100m behind and closing.

Can the 510 do this or is this one of the things only 810 can do??

no one?

I would imagine that it requires each rider to have the 510 linked to their smartphone, which is logging in real time the rider position. Without that link it will not communcate any information as it is Bluetooth only. The 510 has no mapping facility so I guess it could give a distance ahead (as the crow flies) but not in terms of a map, unless yoiu have a pre-recorded course.

Manual is online if you want to read it.

To be honest, for me, I saw no advantage over the 500 other than the facility to store more tham three bikes in the set up. The 8xx series is a different beast altogether, but I struggle to see why the 810 is better than the 800.