Um, no, the inside of the wheel turns the same number of times as the outside. How could it possibly turn more or less times? Swing your arm around three times. Did your bicep swing around more times than your hand?

For maximum accuracy, you should put the magnet and sensor close to the hub of the wheel. Theoretically it doesn't matter, but the problem is that near the rim, the magnet will be moving past the sensor much faster (but the same number of times per minute) and at higher speeds, the sensor may not pick up every time the magnet goes past it. It will start to "skip" and you will suddenly start getting drastically lower speed readings. This may or may not happen with any particular computer, but it doesn't hurt to move the sensor towards the center. As a side benefit, with the magnet nearer the hub, it will be less likely to unbalance the wheel.