Does the fuel gauge read full when the pump shuts off? Rash of defective fuel sender units out there. Also, check your instrument cluster readout for gallons remaining in the tank vs. fuel gauge to confirm these match. Finally, fill the tank, reset the trip odometer and the computer (not the 4 hour instantaneous portion but the total since reset) and drive the car for most of the tank of fuel. Do the math to confirm that the miles traveled on the trip odometer and miles traveled and mpg on the computer match. Form a conclusion from this as to whether the tank is holding its stated capacity. Mark