Originally Posted by CdCf
Place the sensor as far from the hub as possible. And by as far from the hub as possible, I mean where the distance between the magnet and the sensor is the largest possible where the sensor still registers the magnet. This is pretty easy to do - takes no more than a couple of minutes to adjust to perfection.
The point is to make the magnet pass by the sensor as fast as possible, to minimise the time the sensor is "tripped". It could possibly affect the accuracy of the measurement.
It is, of course, true that each revolution takes place simultaneously at any distance from the hub, along the same radial, but the rotational speed is much different, while the magnet's "sphere of influence" has the same size regardless.
Uh wow.
Originally Posted by Sheldon Brown
With one-magnet cyclecomputers, you usually have a choice of several places on the wheel to attach the spoke magnet, depending on the spoke pattern of the wheel. Generally, it is best to mount the magnet as close in toward the hub as possible. The closer in you mount it, the more slowly it will pass by the sensor, giving the sensor's magnetic switch more time to respond. If the magnet is too far out, the computer may give erratic readings at higher speeds.
So completely opposite of what you said basically, and it makes sense actually.