Originally Posted by
ItsJustMe
High end ones might use a hall effect transistor, that would be the smart way to do it. Much more reliable than an inductive loop since a hall effect switch is extremely fast and will give you essentially an on/off switch no matter how slow the magnet is moving (or even if it's stopped altogether). Hall effect transistors are pretty cheap.
It's possible that newer comps have already switched to hall effects, I wouldn't know, my computer is a number of years old at this point.
Don't know of any current bike speedometers that use a Hall effect sensor, but Avocet made some that did back in the '80s. I still have one of theirs which uses a little ring mounted on the spokes at the hub instead of the single magnet used by the reed-switch models. One advantage of the Avocet approach was that it got multiple readings per wheel revolution and therefore a bit faster response at slow speeds.