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Is the gas tank full ? that would make the gauge read full.Have you serviced the cable. a drop of oil can cure a gallon of problems.
The fuel gauge probably has an open somewhere in the sender circuit. With the ignition switched on, jump the fuel gauge sender pin directly to ground. The gauge should rapidly drop to Empty. Check the PCB-to-gauge connections and the I/P cluster harness-to-PCB connections. Burnish the copper foil and terminals at the points of electrical contact using a soft rubber eraser, if necessary. But be careful not to peel the foil away from the plastic substrate; it's an easy mistake to make.Now is a great time to lube the speedometer cable! Make sure the cable is fully seated onto the back of the speedometer head. When you stated that you "used a drill to turn the speedo and it moves like it should," was that with the drill motor attached at the transmission end of the cable? It is very rare for the speedometer drive gear to fail without causing visible wear of the driven gear. However, the drive gear may have shifted axially on the output shaft of the transmission if its retaining clip failed or was not properly installed during prior assembly. Peruse the article, "Common causes of 'bouncy' speedometer needles" for additional insight.