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.