Do you mean the driver's hands in the cockpit view only moves 180 (or 90 forget which, been ages since i played) either way? If so thats not the wheel's fault, it's just that PD hasnt implemented hand-over-hand driver animations yet. If you move to outside view, stop the car and engage rear view (so you see the front of the car), and turn your wheel to 18 degrees then to the full-lock you will see there is a difference.
The wheel's movements are being registered it's just the driver's hands arent able to show you that as the animations don't yet exist in GT5P