When I fill my front tank, the fuel gauge goes WAY above full. While driving, it doesn't move for at least 100 miles and then works it's way down.
When I fill my rear tank, the fuel gauge goes just slighly above full. While driving, it won't move at first, but after 60-70 mile it will slowly drop consistently. While driving with the switch in either postion and the gauge reading anywhere but extremely full, the fuel gauge needle will move quite a bit while going around corners, going uphill/downhill, etc.
A friend's father had three older Ford pickups and told me they all had odd-reading fuel gauges. I think it's just one of those quirks of personality inherent in owning a Ford.