first find out whats broke. disconnect the send wire from the guage, if the guage stays pinned its the guage(ignition key must be on). if it drops to empty its the sender or a short in the sender wire.
pull the sender and connect an ohm meter to the terminals(disconnect the wires first), then move the float. if the reading goes up and down as you move the float its a short, if not its a bad sender.
I looks like my fuel gage is stuck in the full position unless I'm now getting 40mpg. Anyone have an idea how I can fix it?
Check the wires on the sending unit, could be a bad ground, clean and reconnect, If this works spray some corrosion inhibitor on the connections. If that doesn't fix it then it's time to replace the sending unit or gas gauge. pretty easy and cheap to do you're self.
, but now may be a good time to check into a Fuel Management System like 
Linear Mode
