There seems to be 2 basic types of sensors for digital speedos, one type seems to be a simple coil and rotating magnet (2 wire), the other is a hall effect device (3 wire).
The first time I ran with one, I used the Dakota one that was on offer from one of the Instrument suppliers, it was a simple 2 wire thing, it worked ~OK~ above 10 kph, below that, it would seem it was simply spinning too slow to give enough output signal.
I then went over to a hall effect type similar to that on offer from Mini Spares. It works right down to less than 1 KPH, however I find after a days run, the following day, it won't work at all for the first few Km and while it was good when new, I'm now noticing the speedo needle gets a small shake in it, like an old work cable!! I've done just on 10 000 Km with this one.
There is another type I've seen which I must remember to grab one of to try, I've not been entirely happy with either of these.