Ohms. That's what could be wrong. If the Ohms rate is wrong on the new sender then it won't work. Are you using the original gauge?
Your ground or hot connection was probably bad on the original sender, and that's why it didn't read correctly. You might try to test it and repair it since it probably is still better quality than the new one that you put in.
These Guys probably know as much as anyone about correct senders. I bought one of their units for my 6 volt truck and it works great and was only about 50 bucks or so.
