I have read all the posts on bad fuel senders and how to replace them, but I am trying to figure out if I should replace mine. My question is: With a good fuelsender, how many miles do you drive with a full fank of gas before the gage moves off the full mark? I can drive 200 + miles before my gage moves off the full mark. Then I can drive around another 150 or so miles before it reads empty. Is this normal? Then my mileage shows up at around 25 MPG

sounds like a bad wiper on the sender. i believe that you can replace the Wiper Part with out replacing the entire sender

Something is not right Should be more linear, but each will vary a bit. I would pull it out and make sure there is something not causing it to hang up a bit. If you don't see a problem, just replace the wiper.