This is the same thing that happened to me. When I first got my mustang it was at about 3/8 full, so I filled it up all the way, the needle was past full. That was where it stayed too. I though/ hoped that I was just getting really good gas mileage, then when it hit 5/8s it started working. It works when it is getting low and that is when it needs too. It doesn't bother me, so it shall stay like that.