JGSA and Nick (Mickmate) are on the right track. The Centroid sender side-mounted on CSX 4000's, with a bent tube in the tank, require very, very precise adjustments on the sender's two screws for Full and Empty. I had to use a screwdriver used for fixing eyeglasses. However, with the geometry of the sending tube, you'll never get a smooth transition from Full linearly down to Empty.
After many adjustment sessions with wifey in the car calling out the needle movement, the gage goes to Full with about a half tank (11 gallons). Once it moves off the Full mark, I know I'm about half a tank, and have about another 100 miles or so left of driving before having to look for a station. The needle bounces around a bit under acceleration, then returns to the reading after staying at steady speed. Trial and error is the only real way to get it calibrated. Just have to get under the car to do it. A lift helps!
