Actually, it doesn't matter. You can mount the sensor anywhere along the height of the fork leg, from the hub to the rim. This is because when you calibrate the computer during original set-up, you are entering a circumference measurement of your tire, in either cm or mm, depending on which model you own. The computer then multiplies this circumference by the number of rotations, which are the same when measured at any point along the radius of the wheel (the spoke), to determine speed and speed related functions.