Sending units really are that bad all they are are resistor cards with a float arm on them. Resistor cards can suffer from contamination in the fuel and wearing out. Ours seem not to be very accurate I know both of mine take a dive somewhere in the 200 to 300 mile range. The guage cluster has a lot of averaging to componsate for slosh and the tank varations also. Really the fuel guage is just an estimater and you should get to know your car and figure on mileage.

Green 2000 SVT #1166 mods-K&N and Custom heat shield, optimized y pipe, Magnaflow center resonator, DMDhttp://www.geocities.com/savinoss/Cars.html -Wifes car 99 Mystique V6 5 spd