Often the after market ones do not read correctly. This can be due to a rod that is improperly bent, a float leaking or the omens not correct. If it is a original sender I would first check if the float is leaking then check the resistance, it should range from 6.5 to 10 omens across the range. There is some adjustment to the gage but most often the problem is the sender. To check this you can use another sender out of the tank. Hook a ground wire to the frame and the sender wire to the post. Turn the ignition to on and have some one move the sender through it's range. This will show you whether you problem is in the tank or dash. You can just ground the sender wire and the gage should read full. I have found originals that did not read correctly because there was corrosion on the coils in the sender. To fix this you need to carefully open the box on the sender and scotchbrite the coil. Then check the resistance. Good luck, let us know what you find.
Marty
